i just restored my cuda, allnew wheel cylinders, shoes, but used the old master cylinder. i bled the brakes and all was working excellant. then it seemed like to more i used the brakes the more the pedal started not to want to go down. until all brakes locked up. i loosed the lines to the master and fluid came out and released the pressure on the brakes. ive seen where the master cylinder quits and you have no brakes, but i have not seen where the more you use the brakes the more brakes are applied and stay applied. i was going to buy a new master but i thought id drop a line and see what you guys think, thanks for your help

I had the same problem on a 71 Charger, replaced the MC and problem disappeared.

because a rebuilt m,/c is cheap i would try that also. if that doesnt do it i would look at the proportioning valve , the part that holds a residual must be sticking.

unhook the brake lines from the M/C & plug the M/C with 2 brass plugs.If your M/C is good and bled out the pedal will be rock hard & virtually no travel(includes light pressure for 20 seconds and stomping on it). If thats the case when you check it then the problem is downstream.

Make sure the pushrod is right for the master. I bought a f600 truck that did the exact same thing. Turned out to be no free travel of the pedal, the pushrod was too long and the piston was alway compressed part way. Driving it 90-miles home the Brakes would apply more and more, the more it was driven, had to stop and release pressure a couple times to make it home.
-dulcich