Power Disc Brakes not working on hard stop?

Dartman61 said
Hello, I wanted to see if you guys had any insight into my power disc brake operation. I've got a 67 Dart GT 440, 4spd with power disc front brakes and rear drums - stock components. I have a manual proportioning valve to limit the rear brakes. This setup is 10 + years old and under normal driving conditions works fine. However, when applying brakes in a hard stop it feels like the pedal hits a hard stop but the brakes don't feel fully engaged. No matter how hard I press I don't get anymore braking power. I'm wondering if this is a limit of the early stock disc brakes or if I have something else going on? It's been like this since I put the car together and I'm finally going through the car and tweaking the stuff I'd like to work better. The question is this normal and if not what can I try to correct?

Thanks in advance

Some ideas:

1) The original KH disc brake pistons can seize in the cylinders as a result of moisture. You might try putting the car on jack stands and seeing if any wheel can be turned by hand or by breaker bar when the pedal is applied.

2) You mentioned a manual proportioning valve. Is that the original valve that is located in the rear brake line, bolted to the frame below the driver? It's shaped like the state of Texas. That's the only prop valve you should have. The original distribution block does not have an internal prop valve. It's only real function is to add the brake warning light, otherwise you could just use a Tee connector at the MC for the front brakes, and run the rear brake line directly from the MC. Some other model mopar distribution blocks do have internal prop valves, so you should make sure you have the correct block. I don't know what the affect of running 2 proportioning valves would be, but it might be a problem.

3) Or, do what I always do: email Dr. Diff.