Disc Brakes Applying On Their Own

[onelow1967]

I am having issues with the brakes on my truck and have a few questions to see if I can solve the issues. I have a 67 C10 with all new 1972 brake components.

- Tuff Stuff brake master cylinder and booster (OE replacement parts)
- Wilwood proportioning valve for the rear brakes only (installed in-line after the master, mounted under the hood)
- SSBC OE replacement rotors and callipers on the front.

I am running the front reservoir of the master to the front brakes without any valves in-line, basically line out of master, split to the two front wheels for full braking to the front brakes. I am using the adjustable proportioning valve from Wilwood on the rear OE drum brakes in-line mounted under the hood coming from the rear reservoir of the master. The issue I am having is that when the temperature of the truck/motor reaches running temperature the front brakes begin to engage without depressing the pedal and the pedal gets very stiff, they seem to get tighter when the temperature rises, to the point of slowing the truck down greatly. When the truck cools down the brakes seem to release the pressure and everything is back to normal. Now doing many hours of research reading about the mechanics of brakes I found this to be a common problem in some circles of Chevrolet owners. What I read was that sometimes with temperature the tolerances of the master cylinder are so tight that they don’t release all the pressure when you take your foot off the brakes. The solution suggested, which I have tried to solve this issue was to trim a ¼” from the rod between the brake booster and the master. In theory this will let the rod retract further therefore letting the masters return valving to work correctly. I also adjusted the brake pedal linkage so there is no tension or force on the plunger into the back of the booster. This worked for a short time, but the problem came back and I had smoking brakes this past weekend. I am at a loss so I hope I can get some advice on how to solve the issue. Thanks.

David Scade

[GASoline71]

My '72 has both front discs and rear drums going to a prop valve. It's the stock setup with the prop valve mounted right under the master cylinder. Maybe a prop valve is needed for the front?

Gary

[onelow1967]

bump

[cdowns]

some master cyl have residual valves built in for the brakes and if the two ports are hooked up reversed of what they were designed for you can have a problem

[Kerry P]

I had a '72 Corvette with brakes that applied themselves occasionally. Turned out to be the vacuum booster. If you can safely drive with no vacuum to the booster that might help identify or eliminate the booster as part of the problem.

[biggestjohn]

If you increase heat, you increase pressure.

I would look into the routing of your brake lines to see where it is getting hot.

Why have a proportioning valve if you are not adjusting the proportion of braking between the front and rear?