I am attempting to diagnose a problem on an aftermarket GM 1-inch bore brake system (having been added to a classic car for safety purposes).
The braking system is a Corvette-style street rod brake setup, discs front, drums rear. Using silicone fluid. The entire system was purchased as an aftermarket kit and now has been replaced bit by bit without uncovering a particular problem, which is little or no braking power, particularly in the rear. The current project has been replacing the rear wheel cylinders, the last bit of the system that hasn't been re-done; in trying to bleed the system I'm finding little pressure making its way to the rear brakes. Just a bit of fluid is coming out with each purge, and the pedal never gains any back pressure (despite dozens of reps), and the drums can still be turned by hand with full pedal pressure.
Blocking off the master cylinder produces a rock-hard pedal, as I think it should. Vacuum is good. The pedal has no feel at all with the car off. With the engine on, there is just a bit more feel, but still going all the way to the floor, and the pedal is slower to
*rise* after pushing it down. There are no leaks in the system.I am suspecting some issue with the booster and/or the interface between the booster and the master cylinder. The initial install seemed to indicate that there was too much travel out of the booster before the piston engaged; we tried adding some washers around the inner steel piston (small enough not to block the main piston) and that helped a bit, but not much, and doesn't seem like a viable long- term solution.
There is also a brass combination valve in the system that has not been replaced since the original install; these seem pretty foolproof but can something go wrong there?
Thanks in advance for any help.
JGM