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

Mini7

Go Kart Champion
Location
Charlotte, NC
Hey, I think you are onto something here. RPV’s is something I’ve considered but never gotten round to actually doing.
You have done great R&D on this topic and provided the forum a detailed write-up.

Now I’m curious whether it is needed only on the front or both front and rear calipers?
 

emichel6888

Ready to race!
Location
TX
Thanks, no it is looking like just the fronts. I tend to ramble, but if you read my my last reply I just discovered that the rear gap is almost non existent with how I have it setup now. But the front is just about perfect! I have ordered fittings and will take this out and install them up front in the wheel wells as you suggested.

That is why I was getting so much rotation on track, I essentially increased the rear brake bias. :)

I will probably lose a bit of the great pedal feel I have now, hopefully not to much, but at least I won't get rear brake drag. It is very slight, but I discovered it is happening.

The good news is this change will make this modification super cheap and easy to do. Took a lot of searching but I eventually found the right fittings, should be here in a few days, stay tuned.

In the meantime anyone with a BBK please let us know what pad/rotor gap your system has. I will check the gap on these brembo's when I reposition the RPV's.

A wider range of RPV values or even better an adjustable one would be really cool, you could fine tune your pad/rotor gap, and in turn brake pedal response.
 

vwlotech

Passed Driver's Ed
Location
Lexington, KY
Great write up and DIY, I'm interested in the results of fender / wheel well mount set-up, looking forward to the final product ( and parts used, LOL). Prior to the Macan install, were you coming from a PP/R platform? I haven't heard much on this being an issue, except in this thread, regardless, thanks for sharing all of this insight and I will definitely be gathering the parts, just in case I need to go this route.
 

jmason

Ready to race!
Location
Frederick, MD
So I'm curious. The bracket on the wheel well where the flexible brake line joins the rigid brake line is keyed, as is the flexible brake line fitting. The key is to prevent the flexible brake line fitting from turning and thus prevent it from working loose from the rigid brake line fitting. How will you duplicate this feature in your new plan?
 
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emichel6888

Ready to race!
Location
TX
Okay so I took some measurements and removed the RPV's as it was in preparation to install them in the wheel wells. Here are the piston retraction values I found with and with out the RPV's:

Brembo caliper with 2 PSI RPV (pad to rotor gap) .004"
Brembo caliper without 2 PSI RPV (pad to rotor gap) .010" x2 so actual travel is .02"
Stock front caliper (bench test actual piston retraction) .009"
Stock rear caliper with 2 PSI RPV (pad to rotor gap) unable to measure rotor spins freely but virtually no pad rotor gap
Stock rear caliper without 2 PSI RPV (pad to rotor gap) .009"

The Macan Brembo brakes are indeed low drag calipers with more than double the piston retraction, which explains the lower/softer pedal. The good news is the RPV reduces the piston retraction to the ideal range and slightly less than the stock rotors. With the RPV's installed for the fronts it might not be as awesome as what I had, but still a huge improvement with no rear pad drag.

The stock MC is not stepped and from what I can find there is no available larger option. So the only options to brake pedal height firmness is caliper piston displacement, piston retraction rate, adn using these RPV's. The RPV seems to reduce piston retraction by .006, so this option is really only for low drag calipers.

The Stock caliper piston is 60 mm giving a displacement area of 2827 mm and a retraction of .009"
The Macan caliper pistons are 42 mm giving a displacement area of 2770 mm and a retraction of .02" or .008" with RPV
The ST40 caliper pistons are 34 and 40mm giving a displacement area of 2160 mm and a retraction rate of ??? I would guess around .01. This explains why the ST40 would have a firm high pedal.
The ST60 caliper pistons are 30, 34, 34 mm giving a displacement area of 2500 mm and a retraction rate of ???
 

J Peterman

Go Kart Newbie
Location
USA
@emichel6888 thanks for the writeup and really looking forward to your wheel well installation findings! I use the Macans on my daily driver with PosiQuiet ceramic pads and the low/soft pedal is less than confidence inspiring...would love a firmer pedal with higher engagement.

A question: with the RPV's installed, is turning off the hydraulic brake assist with OBD11/VCDS still recommended? Have yet to get around to doing that on my current setup.
 

emichel6888

Ready to race!
Location
TX
Great write up and DIY, I'm interested in the results of fender / wheel well mount set-up, looking forward to the final product ( and parts used, LOL). Prior to the Macan install, were you coming from a PP/R platform? I haven't heard much on this being an issue, except in this thread, regardless, thanks for sharing all of this insight and I will definitely be gathering the parts, just in case I need to go this route.
I have a Golf R so yes. The stock brakes are actually pretty good, the real issue are those brake booster/hydraulic assist settings, they make the pedal feel soft, vague, and non linear. Just changing those settings makes a huge improvement. For street use it is okay but when you are trying to balance the car on the edge, you want a precise firm high pedal. Those settings with some high temp fluid and pads, and the stock setup is not bad at all.

That is why I could not rationalize spending $3000 on a brake upgrade, sure it is nice but do you really need it? However, this Macan Brembo upgrade is so much cheaper and has many benefits so... However as you can see they have this exaggerated piston retraction that makes the pedal engagement even lower than stock, and that makes it a bit low and soft for my tastes. But again it is not horrible, it is just a bit disappointing and you wish it was better.

With the RPV's pressurizing the entire system it was absolutely perfect IMO, I was just thrilled with how well it worked. Unfortunately I did not look closely at the rear pad gap, I just spun the week and gave it a check. So running them on the front only I will lose some of that awesome pedal feel, how much we will see. The measurement numbers look good so...
 

emichel6888

Ready to race!
Location
TX
So I'm curious. The bracket on the wheel well where the flexible brake line joins the rigid brake line is keyed, as is the flexible brake line fitting. The key is to prevent the flexible brake line fitting from turning and thus prevent it from working loose from the rigid brake line fitting. How will you duplicate this feature in your new plan?
If that were true then what prevents it from loosening on the caliper end? lots of calipers (like this one) don't use a banjo bolt and just screw in. If you study the line as you turn the wheel, there is no discernible twisting force as the wheel moves in and out, certainly nothing that will cause the line to work itself loose. I think the bracket is more about keeping movement stress in general off of the hard line and fitting.
But I can easily make a bracket that does the same thing, I was thinking of something like this:

2020-08-02 19_05_07-wheel well line RPV install.png
 

emichel6888

Ready to race!
Location
TX
@emichel6888 thanks for the writeup and really looking forward to your wheel well installation findings! I use the Macans on my daily driver with PosiQuiet ceramic pads and the low/soft pedal is less than confidence inspiring...would love a firmer pedal with higher engagement.

A question: with the RPV's installed, is turning off the hydraulic brake assist with OBD11/VCDS still recommended? Have yet to get around to doing that on my current setup.
Based on the measurements it looks like it is going to work out really well, hopefully I will get it done next weekend. OMG YES, stock or any BBK you absolutely want to do those OBD settings. I did them back when I still had the stock calipers and it was a huge improvement. I have never driven the Brembo calipers without those settings but I can imagine it is even worse with the added piston gap.

Did you see this comment from Bill "my stoptech st40 2 piece rotor kit is firm as a rock once I did the OBD11 mods" So yes, do it.
 

pseudorealityx

Go Kart Newbie
Location
Decatur, GA
Car(s)
2017 GTI SE
So I'm curious. The bracket on the wheel well where the flexible brake line joins the rigid brake line is keyed, as is the flexible brake line fitting. The key is to prevent the flexible brake line fitting from turning and thus prevent it from working loose from the rigid brake line fitting. How will you duplicate this feature in your new plan?
I "lost" that feature when I used the oe replacement rubber Macan hoses, as the keys don't line it. It works fine, and there's no stress at that location. I did use zip ties to keep it from flopping around. Non-issue IMO.
 

jmason

Ready to race!
Location
Frederick, MD
I "lost" that feature when I used the oe replacement rubber Macan hoses, as the keys don't line it. It works fine, and there's no stress at that location. I did use zip ties to keep it from flopping around. Non-issue IMO.
Perhaps. But when I look at a part and think about it's design, I ask myself the question "Why was it designed this way?" There is a reason for every element of the design. The reason could be related to procurement cost, ease and cost of manufacturing, limited source options, or a number of any other reasons, including safety. I guess I'm pretty conservative. If I think that there is a potential safety implication of a design feature, I would be very hesitant to circumvent that feature.
 

billbadass

Go Kart Champion
Location
your moms house
I will get feeler gauges to test/check my own st40 stoptech kit next time i have it apart also.

I'd be wary of actually worsening the braking distances though by changing my front/rear brake bias with this mod if not careful though so just be sure to be careful here, even small changes in F/R brake bias can have big impacts on max braking distances.
 

emichel6888

Ready to race!
Location
TX
I will get feeler gauges to test/check my own st40 stoptech kit next time i have it apart also.

I'd be wary of actually worsening the braking distances though by changing my front/rear brake bias with this mod if not careful though so just be sure to be careful here, even small changes in F/R brake bias can have big impacts on max braking distances.
Great, I would be very interested to know what you find, I would bet it will be around .04" to .05" as that would match the stock fronts and the rear. That also seems to be a standard value for piston retraction.

These RPV's do not affect brake bias at all, once it goes above 2 PSI the pressure is the same front and back. Brake hydraulic systems operate around 700-1000 PSI, so this is a really really small amount of pressure. When I had them in line with the front and rear, the rear pads were practically already touching, so on initial braking and light trail braking the rear was engaging sooner essentially acting like changing the brake bias, but that did not actually happen. If that makes sense?
Ideally you want the front and rear pad/rotor gap to be the same so both ends respond at the same time. So adding these RPV's actually corrects the imbalance (and lower/softer pedal) caused by putting low drag calipers on a car they were not actually designed for.

All these valves do is maintain a very small amount of pressure when you lift off the brake pedal which is useful for controlling piston retraction and also helps with pad knock. Based on my measurements the RPV's (when installed on the front only) will bring the front and rear back into balance, and also fix the lower/softer pedal. The pad knock back resistance is just an added bonus.
 

emichel6888

Ready to race!
Location
TX
To add some objective reference, and for those that don't have these calipers and are wondering what I am on about when I say low brake pedal, I took some pictures for reference. These were done with the engine off and me actuating the pedal by hand:

Macan_Brembo pedal height measurements1.png

Macan_Brembo pedal height measurements2.png


The pedal height is about 5" to the carpet, but full mechanical travel from this reference is about 1.5" to 5". As you can see it is a lot more than just a little extra dead space. The pedal does not really engage until around halfway through its travel. Ideally under full engagement you would like the brake pedal to be about a 1/2" higher than the throttle pedal (or more). With the RPV's installed as I had them initially, the pedal would start to engage at 4.75" and was fully engaged at 4.5" it felt awesome, very tight and responsive. I am sure it will be less when connected to the front's only, but hopefully not by much. I will take measurements so you can see something other than just my subjective opinion.

Those of you with sock or other BBK's show us pedal height engagement points. You have to position the camera just right otherwise you get a lot of parallax error, for example I had to reposition the camera to get a shot of the pedal up and depressed. We know what the pedal up height is so just depressed if you can.
 
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