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How to fix/improve low soft Brake pedal with Residual Pressure Valves and seal lube

emichel6888

Go Kart Champion
Location
TX
How to fix/improve low/soft Brake pedal with Residual Pressure Valves

Let me start by saying that the stock brakes on this car (MK7 Golf R) are fine for street use, but the pedal is soft, and the engagement is nonlinear making it difficult to modulate for high performance driving. Even for street driving, I prefer a firm consistent linear brake pedal. In the Macan/Brembo caliper upgrade thread (link below), some folks discovered that you could edit some of the OBD settings to reduce or turn off the brake boost and hydraulic assist. I initially tried these settings with the stock calipers, and it is a significant improvement IMO. It does provide a firmer more consistent linear brake pedal, although a bit low in the pedal travel, I highly recommend those adjustments.

https://www.golfmk.com/forums/index...brembo-upgrade-thread-read-first-post.371028/

It seems that VW purposely built in brake pedal slop so the ECU has room to regulate initial braking force pedal response using this hydraulic servo assist/booster thingy. That is why the pedal is soft and difficult to regulate (“grabby”). So when you step on the pedal the car stops well, but there is very little feel, and it is difficult to modulate. Apparently, it knows better, just step on the pedal and it will figure out how much braking force you want, for you.

When you change these settings and essentially turn off those functions, you will find that even with the stock calipers the pedal has a much firmer linear feel. However, it is also at a somewhat lower point in the pedal travel because the booster/assist is no longer affecting the initial pedal response. Again, I prefer these brake assists off and highly recommend those settings. BTW these setting do not affect the adaptive cruise control or emergency brake functions.

However, I then upgraded to the Macan Brembo calipers recommended in the thread and immediately noticed it had a somewhat negative effect to the brake pedal operation. In particular, the brake pedal travel before engagement is even lower now and the initial engagement is not quite as firm. Once you reach the engagement point it works okay, and they are linear and easy to modulate, so it is not horrible, just a bit disappointing. I also found the lower pedal will pump up all the time just during normal street use. If I let the brakes rest, with my foot off the pedal for a few seconds, it goes back to a lower pedal, but pump them quickly and the pedal is noticeably higher, does this sound familiar?

I figured it was due to a mismatch of the Master Cylinder to the upgraded calipers, and short of finding a suitable MC upgrade; there was not much to be done, so just get used to it. However, I found in some sudden emergency stop situations the added delay from the longer brake pedal travel is noticeable and can even cause a very brief moment of panic.

I started investigating what was causing this issue, and what if anything I could do to fix it. First, it is a good idea to understand what causes this issue and that starts with what causes the pistons to retract when you let off the brakes. Piston retraction is a function of the inner piston seal, see pic below:


Piston retraction.png

The piston seal lives in a groove in the caliper housing, and is designed to deform a little bit as pressure builds in the caliper before it allows the piston to slide. When pressure is released, the deformed seal retracts back to its original shape, pulling the piston back with it. The deformation allows running clearance off-brake, and the sliding allows compensation for pad wear.

Most brake systems have little to no residual pressure, when you let off the brakes the pistons retract and the fluid is pushed back into the reservoir. When you apply the brakes the MC has to push enough fluid to pressurize the system and move the pistons up enough for the pads to contact the rotor before the car will even begin to brake. To help do this some performance disk brake systems use a stepped bore MC (Also referred to as a Quick take up MC, see pic below). As you can see, at the initial brake pedal movement it uses a larger piston to force a greater quantity of fluid to overcome this brake resting state. Then as you press further, the larger piston is closed off and the smaller piston provides greater force ideally at the point where the pad/rotor is fully engaged.
Master cylinder function.jpg


UPDATE: My original theory on why this works so well involved the MC being a stepped QTMC which I have discovered is not the case. After doing more research I discovered the real reason this works so well. Rather than reposting it, you can read about it here: Why and how it works

All this helps to illustrate why you have to be careful when adding larger calipers with 8 pistons or 12 depending on which BBK you choose. You want to make sure the caliper piston displacement is the same or even less than the stock calipers. Otherwise, you could end up with a much lower softer brake pedal engagement because the stock MC does not move enough fluid to pressurize and move all of those pistons in far enough for quick pad/rotor engagement. This is what can cause the need to press the brake pedal much further down before engagement giving a low and softer initial brake pedal feel. Once you get there it is fine, but by then the travel and pedal response is less than ideal.

Another factor could be the caliper type. There is a so-called standard caliper, or one that is called a “low-drag caliper” (see pic below). We have confirmed that these Macan calipers are in fact low drag calipers. They have very similar displacement to the stock calipers, but because they retract even further, more fluid is required for pad/rotor engagement, and this causes a lower/softer pedal. This is also what causes the pedal to pump up, if you pump the brake the pistons do not fully retract but when you momentarily lift off the brake pedal you pull in more fluid from the reservoir and…

low drag caliper function.png


This is also why you need to be careful about replacing those inner seals, and what material you choose. It can have a much bigger impact on the overall brake performance than you might think. For example, a softer material would distort more causing more piston retraction. So just changing the seals, or worn damaged heat cycled seals can significantly affect the brake pedal engagement, even if they are not leaking. This is why you want to rebuild them periodically with heavy track use, even if they are not leaking.

Here are some values for reference:

The Stock caliper piston is 60 mm giving a displacement area of 2827 mm and a retraction of .012"

The Macan caliper pistons are 42 mm giving a displacement area of 2770 mm and a retraction of .016

The ST40 caliper pistons are 34 and 40mm giving a displacement area of 2160 mm and have standard caliper retraction, I would guess around .01. This explains why the ST40 would have a nice firm high pedal. (Especially if you turn off the brake assist features)

The ST60 caliper pistons are 30, 34, 34 mm giving a displacement area of 2500 mm and standard piston retraction.

For reference:

http://www.remaninc.com/pdfs/lowdragcalipers.pdf

A low drag caliper retracts .020-.025" after the brakes are released. A regular caliper retracts about .005-.010" You would not think that's much, but with a 1" bore master cylinder that .025" on each caliper turns into almost 1/4" of piston travel at the master cylinder just to get the pads to touch the rotor. With a 6:1 pedal that is 1.5" of pedal travel before you even start to build pressure. Add the amount of pedal travel it takes to actuate the rear brakes and take up slop in the linkages, brackets, etc, and the pedal starts getting real close to the floor.
You can see it does not take much to have a big impact on pedal engagement height, as little as .25 MM of piston retraction can make a very large difference in pedal height.



So now that we know what is happening and why, what is the easiest and cheapest way to fix it? A larger or two-step quick take up master cylinder would work great. Unfortunately, I could not find one that will fit in this application, so the answer is a device known as a Residual Pressure Valve (RPV). Product description:

“A residual pressure valve ensures that you will have a high, firm pedal next time you press the brake pedal.

Ever had problems with a low spongy pedal or slow, unresponsive brakes? If so, you may need a residual pressure valve installed in your brake line. A redesidual valve acts like a one-way or check valve to maintain brake system pressure to eliminate the problems of a low spongy pedal. This 2 PSI residual valve is installed inline between the master cylinder and brake wheel cylinders to keep 2 PSI of residual 2 PSI at the wheel cylinder for disc brakes. This ensures that you will have a high, firm pedal next time you press the brake pedal.”

I initially installed these RPV’s on my car right at the MC and I have to say that it exceeded my expectations! The brake pedal was always firm and very high with outstanding modulation. The instant you touch the brake pedal the brakes are right their ready to go! It was just about perfect. This is what it looked like:
20200726_193036.jpg


Unfortunately, I found that the stock rear calipers are not low drag and the added residual pressure was causing the rear pads to drag. Not a lot, but enough that this was not a viable long-term solution. So the alternative was to install the RPV’s in the front wheel wells to only counteract the effect of these Macan low drag calipers.

Does it work? Yes, is it as awesome as when I had them right off the MC? Sadly no, it does get you back to the stock caliper pedal height and eliminates the brake pedal pump up. So you do get a higher firmer brake pedal with good feel and modulation so definitely worth doing IMO. It is just more of a subtle improvement than the night and day difference with my initial modification, oh well. It should also help prevent pad knock which is an issue you can experience under certain track conditions (not on the street).

UPDATE!
As mentioned above, I was not entirely happy with the wheel well RPV, so I did some more testing and tinkering, and… Eureka! The short story is: all you need to do is install the RPV’s as shown below and then just lube the piston seals; that is it! The firm high pedal with instant response is back, I am extremely happy! There is much to explain and more pictures needed, but I am only allowed 10 pics per post so I will add the stage two details into one of my responses further down page one of this thread.

Here is the modification that works without causing pad drag:

Parts needed:

1. 2X 2 PSI Residual Pressure Valve (RPV) https://www.summitracing.com/parts/wil-260-13706?rrec=true

2. 1X Thread Sealant for NPT fitting https://www.summitracing.com/parts/ptx-59214

3. 2X Hydraulic Fitting Metric M10 M10X1 M10X1.0 Male Flat to Flare Steel https://www.fittings.space/gsm23-m10x10m-m10x10mf

4. 2X Male 1/8“ NPT x M10X1 M10 Female Pipe Fitting Connector Adaptor https://www.fittings.space/gbm2p-02zm-m10x10f

5. 2X EMPI 18-1102 BRAKE LINE FITTING x2 MALE 1/8"NPT FEMALE 10MM X 1.0 BUBBLE FLARE https://www.ebay.com/itm/EMPI-18-1102-BRAKE-LINE-FITTING-x2-MALE-1-8-NPT-FEMALE-10MM-X-1-0-BUBBLE-FLARE/264218456228?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

6. 1X 1 in. x 48 in. Aluminum Flat Bar with 1/8 in. Thick https://www.homedepot.com/p/Everbil...m-Flat-Bar-with-1-8-in-Thick-801927/204274000

7. 2X 7/16” to ½” by 1” bolt, nut and washer hardware store

1. $36
2. $5
3. $14
4. $14
5. $12
6. $11
7. $8
Total $100

You will also need some brake fluid, a 28” long 2 x 4, brake bleed setup, drill, metal cutting saw, sander/grinder, wrench…
RPV parts breakdown.jpg



Step by step:

1. Assemble the fittings onto the RPV’s using thread sealant on the NPT fittings. Keep the first 1-2 threads clean, the EMPI 18-1102 fitting is tapered so doesn’t really need sealant. Best to do this the day before and let the sealant cure overnight.
RPV fitting assembly.jpg


2. Jack up the front of the car and remove the front wheels.

3. Next connect a bleeder tube to one of the caliper nipples and loosen the bleeder, then use the 2x4 slide it under the seat and move forward with the 2x4 pushing the brake pedal down like this:
Pedal set.jpg



4. With the 2x4 keeping the brake pedal depressed you can disconnect the hard/flex lines without losing all the brake fluid and remove the clips.

5. Next fabricate your brackets:

A. Cut two 5” lengths of the aluminum stock.

B. Each side requires a slightly different angle/position. What you do basically is stick each end about 1 “ into a vise and start hitting with a hammer to get the angle you want. With the lines disconnected you can test fit to get the right angle, and if you make a mistake you have plenty of extra material so… (see pics for reference).

C. Next mark and drill your holes (a stepped bit works well here). I did drill the lower hole a little small and used a set of files to recreate the keyed hole. But honestly, that is not necessary as you still want to use a wrench on both ends for tightening so…

2020-08-23 18_34_29-Presentation1 - PowerPoint.jpg


6. Then just install the brackets, loosely install the hardline first, then the flex line through the hole, use the clip on the bracket to fixate the flex line, then tighten all the connections. Bleed the front lines, check for leaks, put the wheels back on and you are done.

Final assembly.jpg


Note: See my post further down for effects of using lube on caliper piston seals, it makes a huge difference.

So what is the result of this modification? (See pedal measurement results below) It basically brings the pedal up about a ¼ inch, which doesn’t sound like much but it does have a very noticeable improvement in feel. With this modification the brake pedal never goes below the throttle pedal (which it would do on occasion prior). The pedal is firmer and more consistent; I cannot detect any pump up at all.

Unfortunately it is not the super high/firm pedal I was raving about with the RPV’s at the MC, but is it a significant improvement? Yes absolutely.

I even removed them again just to compare back to back to be sure I was not going crazy (very easy to do). I can say that once I did that I could not wait to put them back in. If you notice your pedal is lower than you would like, and the pump up, this will definitely help.
End result.jpg
 
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jmason

Go Kart Newbie
Location
Frederick, MD
Fascinating! Thanks for the write-up.
 

vj123

Autocross Newbie
Location
The Detroit
Car(s)
Former MK7/7.5 owner
A great summary, kudos to the amount to effort to compile and share the information to the community.

First, it is a good idea to understand what causes this issue and that starts with what causes the pistons to retract when you let off the brakes. Piston retraction is a function of the inner piston seal, see pic below: The piston seal lives in a groove in the caliper housing, and is designed to deform a little bit as pressure builds in the caliper before it allows the piston to slide. When pressure is released, the deformed seal retracts back to its original shape, pulling the piston back with it. The deformation allows running clearance off-brake, and the sliding allows compensation for pad wear.

Another factor could be the caliper type, there is a standard caliper or one that is called a “low-drag caliper” (see pic below). I don’t know if these Macan calipers are low drag or not, you would have to remove the inner seal and inspect the channel grove to know for sure. However, if so that would cause even further piston retraction exacerbating this issue. As it is, I have no doubt that just the added pistons is more than enough to cause this low and initial soft brake pedal. I would be willing to bet that this issue is present to some degree with every BBK using the stock MC.

This is also why you need to be careful about replacing those inner seals, and what material you choose. It can have a much bigger impact on the overall brake performance than you might think. For example, a softer material would distort more causing more piston retraction. So just changing the seals, or worn damaged seals can significantly affect the brake pedal engagement, even if they are not leaking. This is why you want to rebuild them periodically with heavy track use, even if they are not leaking.

Let me also chip the in a couple of points regarding rubber seal and seal groove area.

With strict fleet fuel requirements from governments across the globe, OEMs provide more stringent drag requirements to caliper suppliers. Seal groove design is very complex and people have spend decades of their career on this development.

Also caliper suppliers spend a lot of resources developing / adapting a seal to that particular caliper design. In any case, i would be skeptical to swap it with an aftermarket part as its one of the most critical component in the system.
 

pseudorealityx

Go Kart Champion
Location
Decatur, GA
Car(s)
2017 GTI SE
After replacing the seals last week, I believe the Macan calipers are the "low drag" design. Just my 2 cents. Interesting work. Thanks!
 

emichel6888

Go Kart Champion
Location
TX
Phase two, success!
baby fist pump B.jpg


After adding the RBV’s in the wheel wells there was a significant improvement, but I was not entirely satisfied and thought perhaps I could do better with the right piston or seal adjustment. So I ordered some aftermarket pistons for a Q5 and some red rubber piston seal grease in order to do some more testing.

For those that don’t want to read all the gory details, install the RPV’s as mentioned above and just grease the piston seals and you will be amazed at the improved pedal response, height, firmness, it is almost as good as my failed RPV’s at the MC experiment, only this does not cause pad drag!

I have learned much, for those that are interested; read on.

The goal here is to find the best combination that limits piston retraction without causing pad drag. I removed the calipers and made a sort of test jig that would allowed me to easily measure piston retraction on a bench with various configurations:
Piston retraction testing1.jpg

I installed different piston types in various locations with and without seal grease to gauge the effects of each combination. All tests are with brake fluid in the caliper; I would then pressurize with 120 PSI, release the pressure and measure the gap between the piston and the aluminum block with a feeler gauge. All values are minimum values of all pistons without forcing the feeler gauge. Here are the results:
2020-09-13 15_20_43-Presentation1 - PowerPoint.jpg

I discovered that the inner pistons had significantly less retraction than the outer pistons and assumed that was due to the darker coated pistons having a slicker surface; that was incorrect. In fact, the dark pistons actually have a rougher coating with less slip to the rubber seal. The reason the inner pistons have less retraction is due to the inner/outer piston cup seal channel machining differences. The outer piston cups have the low drag channel, but the inner channels have the standard cut:
Caliper channels.jpg

The inner dark pistons actually adhere more to the seal, so when I put them in the outer position with the low drag cut, that results in even more piston retraction; the opposite of what you want for a higher firmer brake pedal. So, the dark and light piston location does matter, and it is best to install them in the OEM position (dark on the inside and light on the outside).

The aftermarket pistons I ordered actually turned out to be the lighter coating, and the coating is even slicker than the OEM light pistons, which gives even less retraction. However, they did not have the channel machined into the piston face like the OEM pistons, and were the same design as the darker inner pistons. I tried installing them on the outer position with the dark OEM on the inside, they worked okay but something felt a bit off under hard braking, it felt like it was a bit softer somehow? I am not entirely sure what that channel does, but I like it, and since the difference was minor I decided to stick with all the original pistons.

When trying to determine what sort of lube to use, this Red Rubber grease kept coming up as the best option, and what many OEM/aftermarket rebuild kits include for use:
Red Rubber Grease.jpg

Once I realized that lubing the piston seals was the clear ticket to optimizing the piston retractions, I was a bit concerned about the 210-230 temperature rating. We know that under track conditions, caliper pistons can see 500-600 degrees or more, so this would not be a very good solution if the first time the brakes get hot the grease brakes down and/or melts away. I could not find a lot of information about piston seal grease and track use online however, I did find a forum for motorcycle racing; there were several folks with brake issues (mainly pad drag) and they were raving about how this grease on the piston seals eliminated this issue for their track use. So that was encouraging, but I was still a bit concerned and decided to do my own test.

I used the stock caliper, greased up the piston (no dust seal), added brake fluid to the caliper, cranked my oven to 550 F (as high as it will go) and left the caliper in there for over an hour. When I took it out the caliper itself was almost 600 F (oven must be hotter than it indicates) and the brake fluid was smoking/boiling (I was using cheap stuff for testing). I let it cool and tested the piston retraction, and it had not changed at all. Then I drained the cooked fluid and removed the pistons, the grease I had applied look the same, it had not melted or broken down in any way that I could detect. Perhaps that temperature rating is for much longer sustained periods?

How long will it last? We will see, but I also decided to test the stock single piston caliper while I was at it, and that also yielded some interesting information. As you can see, the stock calipers have a piston retraction of .012”, which is a little high but not too bad; better than the Brembo’s combined .016”; adding the 2 PSI RPV also brought it down to .009”. I then removed the stock piston and noticed there was definitely some sort of grease on the piston/seal. So I cleaned it up with some alcohol and just used brake fluid for lubrication (brake fluid is not a lubricant, but better than dry rubber). Then I tested the piston retraction and wow! .024” I then checked the shop manual and sure enough they do specify using a lubricant on the piston/seals:

“The surfaces of the piston and seal must be cleaned only with methylated spirits, and then dried.

– Apply a thin coat of assembly paste G 052 150 A2 to piston and seal before inserting.

– Insert seal in brake caliper.”



This VAG assembly paste/grease appears to be a specially formulated type of lithium grease. NOTE: Most lithium greases are petroleum based and will damage rubber seals, so do not try to use any old lithium grease in this application. The shop manual is very clear that only this particular grease is allowed for this application. I could not find any specifications for this lube, but lithium grease typically has a high temperature rating, so if this red rubber grease ends up not lasting very long this could be a good alternative.



However, after the dry test I then used the red rubber grease on the stock caliper and found that it is a more effective lubricant as the piston retraction went from .012” to .009” with just the red lube. Then I added the 2 PSI RPV and got .006”; which is the same as the Macan Brembo caliper with the same grease and RPV. So this RPV and seal grease modification should work just as well for folks with the stock PP caliper wanting a higher firmer brake pedal.



As you can see below the stock caliper seal channel also has the low drag channel angle, the only thing that keeps it from having a much larger piston retraction is the use of lube. If anyone were to rebuild this caliper and not lubricate the seal, you will be very unhappy with the brake pedal height and feel:
OEM caliper.jpg


In the past when rebuilding calipers I had always just used brake fluid to lube the seals, figuring I did not want to contaminate the brake fluid. With the Brembo calipers you can see they put a slick coating on the cups and the pistons so you don’t necessarily have to use lube. However, the stock calipers have no such coating and the bare metal sticks to the rubber seal so aggressively that you have to use lube in that case.

After this experience I say use lube always! You want the piston to retract so it is not applying pressure to the pad/disk, but any further retraction beyond that does nothing but decrease brake pedal height/response. The use of lube reduces this effect, makes assembly much easier (especially with new tight seals), reduces seal wear, and could prevent piston sticking. I did not take pictures of greasing the pistons but here is an example I found online for reference:
Piston greasing.jpg

Red Rubber grease: https://www.ebay.com/itm/2-oz-100-P...863421&hash=item3f6c313bcd:g:5Y4AAOSwYIxX8Ev8

VAG brake caliper piston seal grease: https://www.ebay.com/itm/362498323200

What are the final results of the 2 PSI RPV’s and lubricating the piston seals? Here is the objective results:
Brake pedal hieght comparison.jpg

The pedal travel goes from 5” to 1.5” where it is bottomed out to the floor, so total brake pedal travel is 3.5”. With the original Brembo install, the pedal went down 1.75” before full engagement, basically halfway down, and the first .75” of travel there is no brake engagement at all (dead space). There is good reason why I went through all of this trouble; it really is too much travel.

As you can see the 2 PSI RPV helped raise it a little over .25” which was a definite improvement but… I was lukewarm on it however, now with the RPV and the lubricated seals?
Yeah.gif


Objectively you can see brake pedal travel went from around 50% travel to full engagement (with a large dead space with no braking at all), to now where it starts braking virtually as soon as you touch the pedal, and full travel is a little over 25%! However, subjectively it is significantly better! It is so much firmer, and the linear modulation (especially under hard braking), trust me, don’t argue, just do it, and thank me later.

I also verified there is no pad drag, by not only sound and feel, the real test was driving on the highway for 20-30 minutes without touching the brakes, pull off slowing with engine braking only and stop with brakes below 5 mph, Jump out and touch the brake rotors, and they should be stone cold. This test has passed so complete success!


The other odd thing I noticed (and really like), you know how when you get in the car, step on the brake pedal and it is firm, then you start the engine and it sinks like half an inch and feels kind of squishy? Well, with these modifications it does not do that, it just stays firm and hard and does not move. When I had the RPV’s at the MC it did the same thing then as well, and I really like it! I have a theory about why this works so well:

UPDATE: My original theory on why this works so well involved the MC being a stepped QTMC which I have discovered is not the case. After doing more research I discovered the real reason this works so well. Rather than reposting it, you can read about it here: Why and how it works

This explains why now when I jump in the car and fire up the engine the brake pedal does not move and remains high and firm. It also explains why when I only reduced the piston retraction another 50% I got a 200% increase in pedal height. It was even higher with the RPV’s at the MC, but this is close enough and gives the same much higher firmer pedal; also now, as soon as you step on the brake pedal the car immediately starts to slow, there is virtually no dead space!

Trust me, you want to do this!

And not just for the Macan, stock calipers for sure, possibly even other BBK’s could benefit as well.
 

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emichel6888

Go Kart Champion
Location
TX
One other thing, I swung by my local track (I have a membership) and just did a few quick laps, the brakes were awesome, so much better. I already have my car setup to over steer (its a lot of fun) but was surprised at how much more rotation I was getting, I might even have to dial it back a bit now. Then it dawned on my why, with the brake response improved that also improve the XDS (electronic limited slip) response which also relies on the brakes.
Just saying, I am really liking this modification.
 

ketoi12345678

Ready to race!
Location
FL
One other thing, I swung by my local track (I have a membership) and just did a few quick laps, the brakes were awesome, so much better. I already have my car setup to over steer (its a lot of fun) but was surprised at how much more rotation I was getting, I might even have to dial it back a bit now. Then it dawned on my why, with the brake response improved that also improve the XDS (electronic limited slip) response which also relies on the brakes.
Just saying, I am really liking this modification.
Do you know if other BBK suffer from this symptom (i.e TTS brake, Stoptech...)
 

emichel6888

Go Kart Champion
Location
TX
Do you know if other BBK suffer from this symptom (i.e TTS brake, Stoptech...)

I don't have those kits so I cannot say for sure, but a lot of folks have looked at this already, and no one has piped in claiming their particular BBK does in fact have a high, firm, pedal that never pumps up, I think that is somewhat telling.

Like I said, it is not horrific, just less than ideal IMO. I am used to cars setup for track use with a high, responsive, hard/firm brake pedal where you can really feel what the brakes are doing. Turning off that brake booster hydraulic assist crap helps a lot, but then the larger piston displacement steps it back a bit with a lower pedal engagement. The RPV's fix all of that, apparently some do not mind a somewhat lower and softer pedal.

Even the stock calipers are a bit low and soft for my tastes, I would recommend this even for the stock calipers so.... At the very least change those booster settings and feel the difference, that gets you closer, this takes it home. I have a DSG so no heal and toe required, but the brake pedal height and firmness is perfect for heal and toe. So if you have a manual and go to the track, even more reason.
 
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emichel6888

Go Kart Champion
Location
TX
I found some additional information that shows how very small differences in piston retraction can make large differences in pedal height and firmness.

http://www.remaninc.com/pdfs/lowdragcalipers.pdf

A low drag caliper retracts .020-.025" after the brakes are released. A regular caliper retracts about .005-.010" You would not think that's much, but with a 1" bore master cylinder that .025" on each caliper turns into almost 1/4" of piston travel at the master cylinder just to get the pads to touch the rotor. With a 6:1 pedal that is 1.5" of pedal travel before you even start to build pressure. Add the amount of pedal travel it takes to actuate the rear brakes and take up slop in the linkages, brackets, etc, and the pedal starts getting real close to the floor.
You can see it does not take much to have a big impact on pedal engagement height, as little as .25 MM of piston retraction can make a very large difference in pedal height.

Another way to tell if you have a low drag caliper is to use a feeler gauge between the pad and rotor, if it is around .20" it is a low drag caliper, if it is .005 to .01 it is standard. However it appears that the low drag caliper is designed to prevent brake drag which can cause all sorts of problems including increased fuel consumption. I have taken some measurements and can confirm that the stock calipers are not low drag and the Macan Brembo are, which explains the lower pedal.
 
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billbadass

Drag Racing Champion
Location
your moms house
my stoptech st40 2 piece rotor kit if firm as a rock once did the OBD11 mods and only the tiniest bit of free play at the top of the pedal, maybe <1/10 of the pedal travel. it is a profound improvement over the stock vw brakes and is now one of my favorite things about this car. and the communication through the pedal with the pagid pads is absolutely crazy, far better than any of the higher dollar sports cars I've owned previously. at first i was worried i might have pad deposits because there was a tiny bit of feeling i could get through the pedal, then I realized I was literally feeling the slots in the rotors through the pads and then through the brake pedal. I'm also using that expensive castrol race fluid.

i am also very very picky and sensitive to how the brake pedal feels on my fast cars and i'm sure this mod you describe above would improve them but this stoptech st40 kit is the best brakes i've ever driven on a street car hands down.

when i turned up the brake pad drying frequency in the rain i did notice a decrease in my mpg fwiw also


one thing i have noticed though is that with this VW if you come off the gas really hard and fast and then immediately go to the brake i think there is some emergency feature where it automatically starts pressing the pads to the rotors so then when your foot gets to the brake pedel in that scenario it is a bit firmer and feels better. i wish i was a wizard with VCDS or OBD11 to make the car do that all the time. (which is probably what your mod here effectively does)

on a side note using titanium shims behind the brake pads made an obvious improvement on reducing heat transfer into the caliper, highly recommend those even for street car if for no other reason than to just extend brake fluid and caliper life
 

billbadass

Drag Racing Champion
Location
your moms house
I found some additional information that leads me to believe all of these calipers, stock, Brembo, and all the other aftermarket kits are all low drag calipers.

http://www.remaninc.com/pdfs/lowdragcalipers.pdf

A low drag caliper retracts .020-.025" after the brakes are released. A regular caliper retracts about .005-.010" You would not think that's much, but with a 1" bore master cylinder that .025" on each caliper turns into almost 1/4" of piston travel at the master cylinder just to get the pads to touch the rotor. With a 6:1 pedal that is 1.5" of pedal travel before you even start to build pressure. Add the amount of pedal travel it takes to actuate the rear brakes and take up slop in the linkages, brackets, etc, and the pedal starts getting real close to the floor.
You can see it does not take much to have a big impact on pedal engagement height, as little as .25 MM of piston retraction can make a very large difference in pedal height.

Another way to tell if you have a low drag caliper is to use a feeler gauge between the pad and rotor, if it is around .20" it is a low drag caliper, if it is .005 to .01 it is standard. However it appears that the low drag caliper is designed to prevent brake drag which can cause all sorts of problems including increased fuel consumption. They were developed in the 80's so what are the odds any manufacturer would not use a low drag caliper? I am thinking slim to none so...


based on this i don't think the stoptech st40 calipers are low drag? when I pull my pads out the back of the caliper they feel like they are basically touching the rotor and no way there is 0.20" of free play in there

i got two piece rotors also because they are lighter and to reduce heat transfer into the bearings etc and
 

Mini7

Autocross Newbie
Location
Charlotte, NC
Car(s)
2017 GTi Sport PP
Any reason why these valves cannot be installed at the end of the hardline in the wheel well with an adapter fitting to the flex line?

It will simplify the installation process.

Thanks for sharing. Great write up.
 

emichel6888

Go Kart Champion
Location
TX
my stoptech st40 2 piece rotor kit if firm as a rock once did the OBD11 mods and only the tiniest bit of free play at the top of the pedal, maybe <1/10 of the pedal travel. it is a profound improvement over the stock vw brakes and is now one of my favorite things about this car. and the communication through the pedal with the pagid pads is absolutely crazy, far better than any of the higher dollar sports cars I've owned previously. at first i was worried i might have pad deposits because there was a tiny bit of feeling i could get through the pedal, then I realized I was literally feeling the slots in the rotors through the pads and then through the brake pedal. I'm also using that expensive castrol race fluid.

i am also very very picky and sensitive to how the brake pedal feels on my fast cars and i'm sure this mod you describe above would improve them but this stoptech st40 kit is the best brakes i've ever driven on a street car hands down.

when i turned up the brake pad drying frequency in the rain i did notice a decrease in my mpg fwiw also


one thing i have noticed though is that with this VW if you come off the gas really hard and fast and then immediately go to the brake i think there is some emergency feature where it automatically starts pressing the pads to the rotors so then when your foot gets to the brake pedel in that scenario it is a bit firmer and feels better. i wish i was a wizard with VCDS or OBD11 to make the car do that all the time. (which is probably what your mod here effectively does)

on a side note using titanium shims behind the brake pads made an obvious improvement on reducing heat transfer into the caliper, highly recommend those even for street car if for no other reason than to just extend brake fluid and caliper life
Hey Bill, Thanks for the great feedback! If you can I would love to know what the piston retraction is with those calipers. I just checked mine with the RPV's and it is about .004" it might actually be .005" my gauge set goes from .004 to .006" and that definitely wasn't fitting. Unfortunately I did not think to measure before installing the RPV install but obviously it was a lot more. I am really curious now so this weekend I may loosen the fitting in front of the valve and see how much of a difference that 2 PSI makes, I am guessing it will be around .02" we will see. Unless someone else with these calipers can take a measurement and report back?
Rotor_Pad gap.jpg

So out of curiosity I grabbed one of the stock calipers, stuck a piece of metal between the pads and actuated it with compressed air and measured the gap/piston retraction. To my surprise they are not low drag calipers. I measured .009" , this actually makes a lot of sense when you consider my results. There is a bit of dead space and softness with the stock calipers which got worse with these Macon Brembo's and then with the RPV's installed this caliper is now much better than stock.
stock caliper rotor pad gap.jpg


It seems that non of the caliper manufacturers seem to post specifications for piston retraction, which is unfortunate as we are now learning how critical it is for proper brake pedal height and feel. I think it would be a good idea to start a database for various calipers, if folks want to report measurements I will create a chart for the initial post as a reference.

I also saw that pad dry setting, but haven't had any brake issues in the rain and figured best to leave it alone. So other than reduced mileage did you notice any benefit?

one thing i have noticed though is that with this VW if you come off the gas really hard and fast and then immediately go to the brake i think there is some emergency feature where it automatically starts pressing the pads to the rotors so then when your foot gets to the brake pedel in that scenario it is a bit firmer and feels better. i wish i was a wizard with VCDS or OBD11 to make the car do that all the time. (which is probably what your mod here effectively does)
I am going to guess that your pad/rotor gap is going to be close to the stock caliper, or somewhere in between stock and where mine is now with these RPV's installed. because my pedal response is just about perfect, I don't think it could get much better without starting to cause brake drag. Obviously there is some dead space but it is so small, definitely less than 1/10, it feels like as soon as my foot touches the pedal I feel the brakes engaging. Full pressure the pedal is slightly higher than the gas, which is what you want for optimum heal and toe. That is if I had a manual and were doing heal and toe, oh well.

Funny you mentioned titanium shims, mine just arrived two days ago. (y)
 

emichel6888

Go Kart Champion
Location
TX
Any reason why these valves cannot be installed at the end of the hardline in the wheel well with an adapter fitting to the flex line?

It will simplify the installation process.

Thanks for sharing. Great write up.

🤨Uh,, dammit Mini7 where were you when I was planning this modification? :D Actually I did consider that option, but at the time I wasn't sure how this was going to work out and wanted to go for maximum effect by keeping a bit of pressure through as much of the system as possible. But now seeing how well it works and having a better understanding of it all, simply putting these inline between the hard line and the flex would be super easy and most likely give 95% of the same benefit.
In fact as I was writing this I realized this is also pressurizing the rear calipers as well, which I checked and all four wheels spin freely. However, I assumed the stock calipers were low drag and as long as they were not touching no worries right? Well I just went out and actually checked the rear pad/rotor gap, and it's not good. They are not actually dragging but the gap is way to tight, like .0005, so when they heat up they probably are dragging a bit, in fact I can see evidence of this on the rear rotors. This is how we learn, now I see why the XDS is working so well, and why I was getting better rotation.
So it looks like I am indeed going to find out how well installing these valves in the wheel well works. Oh well, this is how we learn. looks like all my fancy brake line work is going into the trash bin. The good news is this will make this modification super easy and cheap. All you need is the valves and some fittings, now I need to go order some fittings.
 
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