In my experience, when master cylinders go bad they fail to give enough pressure due to worn seals. I have never heard of one 'locking'.
Do you have a firm pedal? Mooshy pedal? fades and have to pump up? Park brake off? Fluid level change?

I would pull each wheel and check each caliper to see if that is the culprit. I bet ya that there's a bad one. There is alot to look at here.

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)