My 65 C10 has a CPP front disc brake kit instead of front drums. I still have rear drums. The front disc brakes are not fully releasing. When I leave the house they are fine. After 20 miles or so of in town normal driving/braking, I can feel that the front discs are not fully releasing. I removed, opened, cleaned and then rebuilt my NEW 1995 Blazer MC today. All seemed ok inside. I also have a 1995 Blazer booster. I double checked to be sure my pedal push rod was properly adjusted too (it was). If the problem occurs again I’ll be stumped. I’m still puzzled as to the cause.
Martin '62 Chevy C-10 Stepside Shortbed (Restomod in progress) '47 Chevy 3100 5 Window (long term project) ‘65 Chevy Biscayne (Emily) ‘39 Dodge Business Coupe (Clarence) “I fought the law and the law won" now I are a retired one! Support those brave men/women who stand the "Thin Blue Line"! Hug a cop! USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)
Some MCs have internal check valves inside the front port. I've had to remove them.
Do you have an auxiliary return spring on your pedal arm? The weight of the arm itself could be slightly applying pressure.
A plugged or kinked line in the system will do what you described: pedal force is enough to push fluid through, but ambient pressure not enough to allow it to return.
Bet your using a drum brake master cylinder which has residual pressure valve incorporated into the flare seat that the brake line screws up to. Discs only take 2psi while drums take 10psi and the 8# difference will make disc pads drag. I use a #6 (or #8) tap in the center of the flare fitting, screw in a machine screw and pull seat out. Throw away the guts and reinstall the seat then use a 2# residual pressure valve in the line going to the front.
I chose the 1995 Blazer MC and booster after researching options. The 1995 Blazer is a similar weight truck with front disc and rear drums. The hard line size is also the same as a 1965 C10. My brake pedal return spring works very well and I get a full return without sagging. If the problem continues, I’ll change to a different MC and booster.
Some MCs have internal check valves inside the front port. I've had to remove them.
There might be a check valve. I made sure the rear pot (front brakes) return port is not clogged. That return port is very small compared to the other one. Maybe that’s the check valve?
Not sound like a smart A**, but are the front and rear lines of the master cylinder on the right ports? I plumbed one backwards, and it makes the brakes act really weird. If it was one side I would suspect rubber brake line, but both have to be master cylinder. Back off the master cylinder rod just a bit more and see if the problem goes away. Be sure the return spring holds the lever weight off the rod. You could end up with to much free travel, but it will help diagnose the problem.
Good news! I drove the truck a lot today in a variety of conditions and the problem has gone away. I believe there was something clogging either the equalization port or the intake/return port. Either way, the problem seems to be gone. Thanks for the suggestions. I’m sure the pushrod is properly adjusted. The front and rear circuits are connected to the correct ports too.
