Morning Sirs - As much as I hate coming here hat in hand, again, I'm stumped.

My 500 has the stock drums, brake lines and the single master cylinder (MC) with power brakes. I'm not exactly sure what is causing this pressure build up but it is definitely getting worse. Pressure will build up in the lines to the point where all four brakes are engaging, the pedal is rock hard at this point. I bleed off the pressure (from the brake light switch for easy access), and the pedal goes back to normal and the drums release. When working OK I'll have about 1/2" play on the pedal before the brakes kick in.

I've confirmed that the booster works by shutting down, pumping the pedal till the vacuum is gone, then hold the pedal and start the car. The pedal drops about 3/8" so I assume that the booster and bellows are OK. I've changed out the brake fluid completely three different times with no notable change in results. I don't have a proportioning valve (from what I can see) so that can't be it. I'm thinking maybe the return spring in the MC is bad and not letting the piston come back enough to bleed back but that wouldn't explain the constant slow pressure build up would it?

Used to take a couple hours of stop and go driving before my brake lights would come on and stay on till I bled the pressure off. Now I'm down to about 1/2 hour. Note that I don't have a heat shield in the area adjacent to the exhaust manifold, I don't know that there is even one that would go there or if it's an issue. Anyway, I've never had one apart so I'm hunting around for a NOS MC or a rebuild kit but could sure use some advice before I go there. Thanks in advance.

Just swapped out my master cylinder, my first time to change one. I did a little homework and one thing I learned was that the actuating rod must have the correct free play at the the MC piston. If the rod is in constant contact (no clearance) the  brakes can lock up due to fluid expansion. Brakes generate heat in normal operation obviously and this causes the fluid expansion. I don't have a vacuum booster though but the return spring sounds like a possible cause to me.

You have the exact symptoms my Ranchwagon had.  Found the piston was not returning to the stop in the master.  Brakes were fine, good pedal, good stopping with just short trips.  Imagine my suprise when pulling of the interstate and having a high and rock hard padel and almost no stopping power!  Push rod was just a RCH long.  There should be at least 1/16" play in the pedal to booster link and as much again in the booster to master link. 

Easy to check both.  With the system at rest (engine off, booster vacuum up) check the link on the brake pedal.  The oblong bolt should "fall" into place with everything sitting.  Next loosen the nuts holding the master to the booster, look into the master and you should be able to see the piston through the holes in the bottom.  Push the master up against the booster, there should be no movement of the piston or pressure to put it in place tight against the booster.

Well, there you go. I knew somebody would have seen this. Appreciate the info, I'll get that sorted as soon as I get back home. Thanks a million.

Update - That was the problem... a minor adjustment to the eccentric and it all came together. Thanks Ford Blue Blood and lalessi1.