I recently went through this on my 79 B.
I thought, as did my mechanic, that he had rebuilt the master.
I could not get the rear brakes to work at all.
Eventually, he just replaced the master cylinder with a different, new unit.
Brakes now work, but, just like before, the pressure differential switch is
hanging limply somewhere in the engine bay.
Before I took the brakes apart to do the rear end swap, there was just a bolt in
place of the switch.
YMMV, but I'd say just go for a new matster, don't chance it.
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