Before you condemn the master cylinder, adjust the pedal linkage so that when you look in the reservoir, you see a little spurt of brake fluid. That means the relief port is not blocked. If you don't see that spurt, that means the port is blocked and expanding brake fluid (from heat) has nowhere to go.
Thanks, I will check that out. It's possible that a piece of rust could be blocking the port.
I just went out and check, while pumping the pedal and there is no spurt in the MC, at all, and the bottom of the MC is rusty looking.
