I have a 70 Charger with power disk brakes. The valve setup is the after Jan 1 version shown here:

https://board.moparts.org/ubbthreads/show...rue#Post5434077

I have two problems with the brakes. First the rear brakes skid much too easily when stopping and second the fluid in the master cylinder "transfers" from the large reservoir to the small reservoir over time. If I drive the car a few hours the change in levels is noticeable. The large reservoir goes down and the small one goes up. The master cylinder is a parts store rebuild. I'm thinking that my problem is in he combination safety / proportioning valve which is original to the car. Anybody have any ideas?

it sounds to like the secondary lip seals are week and when applied the primary pushes past causing less pressure on the fronts and more on the rear,and filling the small res.. my guess is master cylinder.

I was thinking because the master cylinder was a rebuilt that it was more likely that the problem was the original safety/proportioning valve but I've had other rebuilt stuff bad from day 1 before so it's quite possible that it's the master cylinder.

I have a question on the safety switch. If I have one side of the system open (say the front) and step on the brakes there should be electrical continuity between the wire stud on the switch and the car body, right? Basically the way the switch works is to ground the wire when there is a big pressure differential between the two sides of the brake system, right?

The prop valve does very little "work". The only time it really moves is when there is a problem with a pressure differential. Seems highly unlikely that it would cause the problem you're seeing with the master. Definitely sounds like a bad master to me. You are correct about the switch continuity if one side or the other of the system is disconnected. I had a problem with my new prop valve when I bought it, it was seized up in the wrong position. This also caused the continuity, but all the time. Needed to take it apart and clean the internals. Would have been better off to reuse the original.