Disc Brake Master Cylinder With Drum Brakes?

A 60s-vintage drum brake MC has a residual pressure valve in the brake line orifices... Your new disc MC will not have one in the front chamber (it is only used for drums, not discs — this is also why you don't use drum MCs with discs). Since the purpose of the RPV is to ensure the brakes apply instantly, you may find yourself in a situation where the rears respond more quickly than the fronts. This is not good. I would not recommend it.