My'69 Malibu's (w/all drum brakes)brake pedal just started to go to the floor. There are no signs of leaking and no fluid is on the inside of the tires anywhere.The master cylinder remains full.Bad master cylinder? If so I plan to change the front brakes(at least) to disc.Any suggestions on new master cylinder size,type.I'd prefer to buy a new master cylinder now that will work with discs later.

I had a similar problem but was intermittent. No loss of fluid, no leaks. Bought $15.95 master cylinder never happened again or rather hasn't happened since last August.
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Sounds like the master cylinder seals are bad. Relace the Master if nothing else looks wet. I would buy a new one from a reputable source like Wagner Bendix etc.

New Master Cylinder fixed the problem!!!Best pedal since I finished the restore(well it will never be truly finished)