Residual Valve Question for the brake experts....

[DonohurJav]

Hi all,
Hoping to get an expert opinion on whether or not to use residual valves & what size if needed on my application.
I'm using Wilwoods new Aero 6 & aero4 calipers with piston areas of 4.04 sq.in & 1.98 sq.in respectively. I'm also using CNC brakes' dual master cylinders with balance bar in a 3/4 & 7/8 sizes as recommended by wilwood. I've studied a couple of brake engineering books as well as most of the posts from Tobin and OG, Ron Sutton etc. and still can't quite grasp when and when not to use residual valves. The plan for this car is 80-90 % street and some autocross with the occasional track day mixed in. For the street I realize I wont have a problem, but for autoX and the track days I know that pad knock-back will be a problem as I am not running a full floater rear. I guess my real question is, will running residual valves keep some pressure on the pistons and help with knock-back? My master is going to be on the firewall in the stock location ie. above the calipers, so I wasn't planning on running any residual valves. I am currently plumbing the system from scratch with the usual 3/16 with braided soft line and would like to do it only once, LOL! If residual valves are recommended for knock-back (or other reasons) , what sizes? 2, 5, or 10lb? Thanks in advance for any and all help! I love this site!

Sean

[DonohurJav]

I should have proof read better. To clarify, I am plumbing 3/16 hard line with braided hose ends like normal.

Sean

[dontlifttoshift]

I'm not an expert but here is the way I see it.

10 pound residual valves for all drum brakes to prevent the seals in the wheels cylinders from relaxing and drawing air into the system.

2 pound residual valves when the master cylinder is mounted lower than the calipers to prevent drain back from the calipers.

***Some caliper mfgs (not wilwood) recommend a 2 pound residual to maintain pedal height due to a seal design that retracts the pistons.

Any other use of a residual valve is a band aid. If you are "solving" a knockback problem with residual valves, you are doing it wrong.

[Ron Sutton]

I'm not an expert but here is the way I see it.

10 pound residual valves for all drum brakes to prevent the seals in the wheels cylinders from relaxing and drawing air into the system.

2 pound residual valves when the master cylinder is mounted lower than the calipers to prevent drain back from the calipers.

***Some caliper mfgs (not wilwood) recommend a 2 pound residual to maintain pedal height due to a seal design that retracts the pistons.

Any other use of a residual valve is a band aid. If you are "solving" a knockback problem with residual valves, you are doing it wrong.

I agree with Donny.

I'll add ...
A. If you really want to eliminate pad knock back at the source, you need to reduce the spindle/hub deflection that is happening from high cornering loads. In the rear ... dual bearing floater hubs are the solution. In the front, spindles with bigger snouts & larger, stronger hub bearings are the solution.

B. If you want to want to eliminate pad knock back at the caliper, like most OEM factories do, you need to run floating calipers.

C. If you are running solid mounted calipers ... on weak OEM front spindles with small hub bearings & rear OEM style flange axles with one bearing per side ... you are going to have pad knock back if driven hard in the corners. Running larger rotors just amplifies the knock back problem.

D. Running a 10# residual valve is a band-aid to "reduce" the pad knock back. It doesn't eliminate it. But it does reduce it. This fix is not without side effects, as there is now 10# of pressure ALWAYS on the brake pads, causing excessive pad & rotor wear, excessive heat & frankly increased drag. So that's not the right way to cure it.

:cheers:

[sccacuda]

Not being a smartass, but I know I'm not the only one that pushes the pedal before a braking zone. That is just the way I was taught and my interpetation has always been to have the the pads set against the rotor. I've had others tell me they were making sure it had brakes, but if you don't, then what. Still running off or crashing at about the same time, so I'll stick with setting the pads as the reason I do it.

Unless this an an autocross issue. Never done any small courses, so maybe you don't have time to set the brakes?

[mitch_04]

Would a 2lb residual pressure valve cause any problems?

[DonohurJav]

Thanks Donny and Ron,

that's kind of what I suspected, but just wanted some more experienced minds to confirm my thoughts. I personally think that i would want as much piston retraction as possible, but just wondered if people were using any in their setup. I could see maybe in a dedicated race car on a short tight course it might have some benefit, but by then , those folks would probably be running the large hubs and floater axle setups etc.
I am in the middle of plumbing my brake lines up and didn't want to miss out on something that I should have done.

Craig,
No offense taken to your reply. I had heard/read as much. Most experienced racers just get used to it and use pedal technique to deal with it . If I ever push my car hard enough to experience knock-back, I'll pedal accordingly. Just wanted to give myself the best setup possible on the first try.

Thanks for the insight fellas!

Sean