MM,
The only thing I can think of is that maybe, just maybe, the front one way valve in the front is in basackards. But, I haven't thought this through enough to be positive about this. Did you rebuild the unit yourself? If so, I would check that all the internals in the master cylinder are in proper order and turned the right way. Mercy!
I'm still thinking. It's gotta be something simpler than what I suggested.
Don't tear it apart yet. Maybe someone else has experienced the same thing, but I never have. Let's hope you don't have to tear the MS apart.
As soon as Gene, Chipper, Tiny, Eastern Ed, MT, JD and a few others wake up this morning (They need their beauty rest), I bet you'll get a lot better answer than mine. I hope so.
Good luck,
Charlie