The "red onion" seems determined to get back on jack stands and keep me from working on my other project teener. Suddenly lost brake pedal pressure this evening after running fine earlier today. I can get pressure back to some degree by pumping the pedal, and I don't see any fluid leaking anywhere. When I got back home I felt all four rotors, and they were all warm (fronts more than back as to be expected) suggesting all brake circuits/calipers are working. One or twice on the way home the warning lights started flashing and then went off.

It's a '72 1.7L daily driver. All calipers were rebuilt last year. Is there anything else I should check at this point before assuming the master cylinder needs to be replaced? I won't get into the 17mm vs. 19mm thing...

check the reservoir

I've got fluid in the reservoir. It hasn't dropped at all.