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How to fix BBK low/soft pedal with Residual Pressure Valves

emichel6888

Ready to race!
Location
TX
UPDATE: I have discovered that this modification is causing to small of a rear pad/rotor gap, so don't do what I did here. The good news is this development makes this modification much easier and should achieve similar results. I plan to remove my fancy plumbing work and just install the RPV's between the hard line flex line connection in the front wheel well. This should achieve similar results without affecting the rear calipers, and make the modification extremely cheap and easy to do. Live and learn, I will update the post ASAP.

Let me start by saying that the stock brakes on this car 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, providing a firmer more consistent linear brake pedal. I highly recommend these adjustments.

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

However, I then upgraded to the Macan Brembo calipers recommended in the thread and found it had a somewhat negative effect to the brake pedal operation. In particular, the brake pedal travel before engagement is really low (almost halfway down) and the initial engagement is not very 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 low 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. 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. Spoiler alert I did find a way to fix it, and it works perfectly! 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 most performance disk brake systems use a stepped bore 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


Our stock MC seems to have a similar design however, it is designed to take up the slack of two pistons, and now we just added 8 pistons or 12 depending on which BBK you have. The initial larger piston volume of the stock MC does not move enough fluid to pressurize and move all of those pistons in far enough for pad/rotor engagement before switching to the smaller high-pressure piston. This is what causes the need to press the brake pedal much further down before engagement giving a low and softer initial brake pedal. 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 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.

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 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.

So now that we know what is happening and why, what is the easiest and cheapest way to fix it? 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 installed these RPV’s on my car and I have to say that it exceeded my expectations! Now my pedal is always firm and high with outstanding modulation. The instant you touch the brake pedal the brakes are right their ready to go! And yet they still retract just fine, there is no drag whatsoever. IMO it is now just about perfect!

Sorry if you thought you were done and this hands you yet another project, but trust me, you want this modification. I would say even with the stock calipers it would be a great modification, but if you have a BBK, I would say this is a must do modification. It is not exactly an easy modification, but perhaps seeing what parts I used and how I did it will help, read on.

UPDATE: I have discovered that this modification is causing to small of a rear pad/rotor gap, so don't do what I did here. The good news is this development makes this modification much easier and should achieve similar results. I plan to remove my fancy plumbing work and just install the RPV's between the hard line flex line connection in the front wheel well. This should achieve similar results without affecting the rear calipers, and make the modification extremely cheap and easy to do. Live and learn, I will update the post ASAP.

Parts needed:



  • $36
  • $12
  • $16
  • $6
  • $23
  • $8
  • $32
Total $133


Special tools you will need (If you’re planning a DIY):

I already had these tools from previous projects, but if you do not have any of these tools the cost is around $179 to $295 depending on which flare tool you want to spring for (I highly recommend the Eastwood tool if you can swing it).

Of course, you will also need some brake fluid, a 28” long 2 x 4, and a 13 mm and 11 mm flare nut wrench. The space is very tight for the back line and I ended up having to cut two of these wrenches in half in order to have room to tighten the fittings in the confined space. If you start with the back line and tighten it well before installing the other connections you might not have to do this, but if you have a leak like I did and need to tighten one of those back connections when it is all together, this was the only way I could get in there.

This is the finished product:
20200726_193036.jpg


Here is the parts put together, you will be making two of these (with slightly different dimensions):

RPV line part breakdown.png


20200726_193029.jpg


20200726_193104.jpg


20200726_193120.jpg


Step by step:

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

  • Remove the air box and air inlet hose to the turbo.
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:
20200726_125808.jpg

  • With the 2x4 keeping the brake pedal depressed you can disconnect the lines from the master cylinder without losing all the brake fluid.

  • Now you can start fabricating your RPV lines, you will want to start with the back line closest to the firewall, that is the one that is hardest to access. You can see the general shape and routing from the pictures. Start with the piece connecting to the MC. Once you get the general shape and position lightly snug the fittings together, minor fine tuning of the bends can be done by hand. The key is to make sure all the lines have at least a 1/4” clearance, you don’t want anything rubbing/touching on any of the brake lines. The only spot I could not get at least a ¼” of clearance was on the back line union to the stock line, it was right next to the MC. It is hard to see in the pictures but I split a piece of fuel line hose and tie wrapped it as a line protector in this area:
Inked20200726_124303_LI.jpg

  • Get the back line done and tighten the connections more than you think you need to. Because if you don’t and find a leak with both installed it is really difficult to access those connections.
  • Once you have both lines fabricated and installed with tight connections, bend the lines and reposition for as much clearance as possible. As you can see I made a little bracket to help hold the valve lines from moving around. While this is not absolutely necessary, it is highly recommended.

  • Then I loosened each fitting at the RPV (which is easy to access) and removed the 2x4, then used the pressure bleeder to push fluid from the MC and from one of the caliper bleeder screws until I saw fluid coming out of the loose fitting. I did this to try and purge the lines of air. I am not sure if this was worth doing because I still ended up doing a full ABS brake bleed anyway so...

  • Now tighten all the fittings and using the pressure bleeder check for leaks and tighten fittings as needed. There is almost always a leak somewhere no matter how well you think you tightened everything. Then have someone step pump the brakes hard to check for leaks again.

  • Despite your best efforts there will still be air in the system and you will need to do an ABS bleed. This is why I say you need a pressure bleeder. The only way I know to do this is to pump up the pressure bleeder to 30 PSI and bleed from both front calipers at the same time. So you will need to bleeder tubes and plenty of brake fluid in the pressure bleeder. Use the cheap stuff for now, you can always replace it later with the expensive high temp stuff after you have resolved all leaks and got all the air out of the system.


I have read that most disc brakes won’t start to drag until the residual pressure is around 12 PSI, so 2 PSI is no problem at all. Yet it is enough to make such a huge improvement in the pedal response. It also helps to reduce pad knock back. To that end Wilwood also has a 4 PSI RPV expressly for that purpose.

“Wilwood is excited to introduce a new 4 PSI-rated Residual Pressure Valve (RPV) to their line of pressure and flow controls. The 4 PSI Gold valve adds an option between the existing 2 PSI Blue and 10 PSI Red valves on disc brake systems where extreme shock loads, hard vibration, and suspension component flex can contribute to excessive caliper piston retraction and pad knock back.”

So far I haven’t had the need, but if that becomes an issue it would be easy to swap these in. This is IMO a better alternative to using expensive, noisy, high maintenance floating rotors.
 
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jmason

Ready to race!
Location
Frederick, MD
Fascinating! Thanks for the write-up.
 

vj123

Drag Racing Champion
Location
The Detroit
Car(s)
2019 GTI Rabbit
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 Newbie
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

Ready to race!
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

Ready to race!
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

Ready to race!
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

Go Kart 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

Go Kart 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

Go Kart Champion
Location
Charlotte, NC
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

Ready to race!
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

Ready to race!
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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