Mike,

I'm currently doing a tandem MC conversion on my '36 Chevy pickup:

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1196340/2.html

In researching that subject I've seen the condition you describe. I'm a geologist, not any kind of vehicle "expert". BUT I've read that one cause of the symptom you describe is fluid from wheel cylinders and/or calipers draining back to the master cylinder. In theory that can only happen when the MC is BELOW the level of the wheel cylinders and/or calipers. If that is what is happening to you, residual pressure valves will be a fix.

In my application I'm using a 1970s era Jeep drum/drum MC in the same under floor location as the original '36 Chevy MC. The information I've found concerning the need for "hold off" valves, proportioning valves and residual pressure valves is all over the internet, but unfortunately also very contradictory.

Maybe we could both work on this issue and the first guy who finds what actually works can tell the other.

Ray