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I just replaced two leaking rear calipers on my 74 (350-power brakes) and went ahead and replaced the master cylinder (rebuilt) while I was at it. I figured for $ 15.99 it would be a good idea since the old one had some rust. I did bench bleed it before installation. After completly bleeding all wheels, I took it for a short test drive. The pedal was firm and the car was stopping. Then the pedal just gives and goes to the floor with no pressure at the wheels. I was just in my neighborhood so I went home and re-blead each wheel again (no air present). There are no leaks and the m/c is still full. Did I get a bad m/c? (I have a Mercedes that acted the same way when the m/c went in it).
I have experienced this and have also read here many times where people go through 2, 3 or more rebuilt master cylinders to get a good one. Just buy a new one, do it once and don't have to do it again. (and again, and again) :yesnod:
That and you don't want to be taking any chances with your brakes anyways! :nonod: