'65 Chevy Truck Power Brakes

[rt65dh]

OK, I'm going nuts...replaced both rear drum wheel cylinders, bled them, same thing. Soft pedal when the motor is off and still drops to the floor with the engine running. Could it have to do with pedal height? When I start it, when running the wheels spin slowly, when I step on the pedal it goes to the floor, but wheels stop. Could the proportioning valve be bad? Doesn't make sense that I plug the MC and the pedal is as hard as a rock...the minute I hook up the lines the pedal is soft. I can't help but think the MC is bad. The system is too simple for this much pain!

[CPPAaron]

Still sounds like air in the master! Pull the master back off and do another bench bleed... if there is air in it you'll never be able to get it out without bleeding on the bench again.

[rt65dh]

Thx for the help. I bench bled it again...same thing. I will go buy a new MC tomorrow. I have wasted too much time on this.