Brake problems on a 1989 silverado?

I have fluid bubbling back up in the small reservoir of the master cylinder whenever you start the engine and the pedal goes down. I have replaced the rear wheel cylinders, master cylinder, brake booster, and bled the lines 5 times. there are no leaks. It has rear anti lock brakes and I was wondering if the anti lock brake valve is what holds the pressure in the line or do I have another bad master cylinder? I even bought a new one, not re-manufactured