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



## emichel6888 (Nov 17, 2018)

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.

I did this modification on a MK7 Golf R but most of it is applicable to all cars so figured I would post it here.

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 full 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:







[/URL]Piston retraction by Eric Michel, on Flickr[/IMG]

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.








[/URL]Master cylinder function by Eric Michel, on Flickr[/IMG]

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.







[/URL]low drag caliper function by Eric Michel, on Flickr[/IMG]

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:


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


4X - RPV interface 1/8 - 27 NPT to 7/16 – 24 inverted flare fitting https://www.summitracing.com/parts/edd-124420
 


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


4X – 7/16 –24 ¼ inch BLF-13C-5 https://www.autozone.com/parts/brak...ting-blf-13c-5/835075_156898_3048_90188_92959
 


4X - BQ9169 Brake Line Fitting Male M12 x 1.0mm 1/4 DIN http://www.clarksvillefasteners.com/index.php?route=product/search&filter_name=BQ9169
 


2X – M12 x 1.0 bubble DIN union https://www.autozone.com/parts/brak...NC4lw9kx9avAZ2tBn5RoCktMQAvD_BwE&gclsrc=aw.ds
 


¼ Inch SS brake line https://www.amazon.com/gp/product/B07BN2W84R/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
 


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

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


Brake line pipe cutter https://www.homedepot.com/p/RIDGID-...ERCH=REC-_-pipsem-_-100075014-_-100021967-_-N $15
 


Brake line tube bender https://www.homedepot.com/p/General-Tools-Compound-Tubing-Bender-153/301369630 $26
 


Flare Tool (Bubble/DIN and double flare) https://www.summitracing.com/parts/otc-6502 $84 This Eastwood tool is pricey, but so much easier to use and gives perfect flares: https://www.eastwood.com/professional-brake-tubing-flaring-tool.html $200
 


Pressure Bleeder (what I have) https://www.ecstuning.com/b-motive-...o6Cvo8ZKf5M7Bb4RKgYgZfflIVQDlaJBoCM9EQAvD_BwE $54
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:







[/URL]20200726_193036 by Eric Michel, on Flickr[/IMG]

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








[/URL]RPV line part breakdown by Eric Michel, on Flickr[/IMG]








[/URL]20200726_193029 by Eric Michel, on Flickr[/IMG]








[/URL]20200726_193046 by Eric Michel, on Flickr[/IMG]








[/URL]20200726_193104 by Eric Michel, on Flickr[/IMG]








[/URL]20200726_193113 by Eric Michel, on Flickr[/IMG]


Step by step:
1. Jack up the front of the car and remove the front wheels.

2. Remove the air box and air inlet hose to the turbo.

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:








[/URL]20200726_125808 by Eric Michel, on Flickr[/IMG]



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:








[/URL]Inked20200726_124303_LI by Eric Michel, on Flickr[/IMG]

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.


----------

