Unusual brake pedal problem

[CanyonKiller]

I put c6z06 brakes on my car a few months ago. When I installed them I also put a new wilwood 7/8 manual master cylinder, new brake lines, kore3 stainless flex lines, and a wilwood distribution/porportioning valve. I used a vacuum bleeder and then a manual bleed on the car. I drove it and noticed that randomly the brake pedal would get rock hard or feel real soft. I had trouble putting the rod for the master cylinder on the pedal so I shimmed the master from the firewall so I had some adjustment and it has been fine ever since.

Last week I took it for a drive to cruise night and as I was backing up the pedal went soft again and I had to pump the pedal to stop. As soon as i put it in park I pressed the pedal again and it felt fine. I let it sit while I walked around and when I came back I looked for a leak and could not find one. It has been fine ever since.

What could be causing this problem? I dont want to be driving it this way as it may cause an accident. Any ideas?

[andrewb70]

What kind of rear brakes do you have?

[Ron Sutton]

Sounds like another case of needing to add residual valves inline. Would you agree Andrew?

[andrewb70]

Sounds like another case of needing to add residual valves inline. Would you agree Andrew?

It's very possible, which is why I asked about rear brakes. I am pretty sure the Wilwood MC doesn't have any sort of valves in it.

Andrew

[Ron Sutton]

It does not. Wilwood does not build residual valves into their Master Cylinders.

[CanyonKiller]

The rear is c6 z06 as well. I was under the impression that residual valves werent needed if the master cylinder is the highest point. So a couple residual valves in the lines from the master to the distribution block should fix this?

[Ron Sutton]

Chris, I am assuming you took out the stock distribution block when you installed the 7/8 M/C. Usually, the distribution block has residual valves built in ... 2# for discs & 10# for drum.

Typically, street cars need residual valves, so the brake lines keep a micro amount of pressure to the brakes (so you don't have to pump them after long periods of none use. We don't use residual valves in race car brakes (Some people do. We don't). We do use them in all street car applications.

Drum brakes need a 10# residual valve & discs only need 2#. This just keeps your brakes "ready" & consistent. If you removed your distribution block, you will need to add residual valves inline. I suggest you install two 2# Wilwood residual valves inline, just outside the master cylinder ... 1 for the front brakes & 1 for rear.

You may, or may not, also have an air in line or leak issue ... but you need the residual valves anyway.

[CanyonKiller]

Yes I did remove the factory distribution block for the wilwood one. I will pick some up from wilwood this week and give it a shot. Thank you for your help.

[dontlifttoshift]

What factory application uses any residual valve other than a 10lb for drums? A residual valve on a drum brake serves one purpose, it keeps the lip seals loaded so the wheel cylinders do not draw air. On disc brakes, the residual valve is to keep the fluid from draining back from the caliper if the MC is lower than the bleeder screw. Any other use of a residual valve is a band aid at best.

If the pedal goes soft and then comes right back with out bleeding it, you are pushing the pistons in on the caliper. Whether it is pad knockback on the front or rear, due to loose bearings or axle endplay or I have seen where the calipers or brackets become the "steering stops" and that will push the pistons in as well.

[andrewb70]

Since you are using a fixed caliper on the rear, I have to believe that you're experiencing pad knock back, as Donny mentioned.

Andrew

[CanyonKiller]

Is there a way to know for sure of its pad knock back? Maybe pushing the car side to side in the rear as hard as I can and seeing if the pedal is soft?

[andrewb70]

Is there a way to know for sure of its pad knock back? Maybe pushing the car side to side in the rear as hard as I can and seeing if the pedal is soft?

That's not a terrible idea...give it a shot.

Andrew

[Apogee]

Is there a way to know for sure of its pad knock back? Maybe pushing the car side to side in the rear as hard as I can and seeing if the pedal is soft?

Generally speaking, if it only occurs after moderate to hard cornering, then it's knockback. How much axial endplay do your axles have? Stock 10-bolt?

If you're randomly getting pedal height fluctuations, then it could be the MC bypassing fluid past the piston seals under pressure for some reason, which is usually an indication of a seal or bore issue. Is the master cylinder new or used? If you pump the brake pedal and hold steady pressure, does the pedal sink at all over time? If so, I would suggest that you have the MC rebuilt.

An inconsistent issue is going to be the hardest to track down, however I seem to remember a thread with a similar issue a while back, I just don't remember if it was resolved or not. I'll see if I can search it out, or maybe someone else will remember it and chime in.

Tobin
KORE3

[CanyonKiller]

I shook the crap out of my car and the pedal felt fine after. I also turned the wheel lock to lock and still a good pedal. It is a stock 10 bolt. I am unsure of how much end play the axles have but I will assume a good amount due to being the original axle and bearings. A 9'' is in the future though. I never notice it after taking corners at speed. It seems to only happen when making slow speed turns or putting it in reverse.The master is new and the pedal does not sink when held under pressure. It also does not feel spongy like there is air trapped. I am well aware of intermittent problems with cars and how aggravating they are as I do this for a living.