Almost sounds like your master is too small. The first one may have been damaged or got something in the piston cup to cause the master bypass and go bad but the replacements do not have enough volume to pump up the brakes. After a couple quick pumps does the pedal come up?

I've always heard but never experience that if you push too much or too hard when bleeding the master you could damage it.

Another thing could be a big air buble in the system that got pushed in when the master was replaced. On some cars this air can be a total pain to remove with out pressure bleeding. Are you using a power bleeder or manually bleeding it?

Good luck

Allen Here's a novel idea, let's not throw a bunch of parts at the car hoping it will fix the problem and instead spend a little time diagnosing it first. Life was a little easier when I was just a wrench.