duster rear disc brakes

If the front/ rear bias (proportioning) is OK, you COULD remove/ destroy the residual valve in the master



If yours originally had front disc, there will only be one valve in the front section which feeds the rear (drum) brakes. For rebuild, "they" used to give you a little sheet metal screw in the kit. You threaded the screw into the fitting, and used a claw hammer to extract the seat and valve parts

On my 67, which now has 4 wheel disc, I'm actually using the original all drum cylinder. I just stuck a nail deep into the master fittings to puncture the residual valve rubbers