Disc Brake Master Cylinder With Drum Brakes?

Without the prop valve to shut off rear pressure at "x" lbs. there would be the chance of rear lock-up under heavy braking, particularly in the rain.
Most modern wheel cyls have the spring in them to replace the residual valve in holding cups against wall .
If you have an adjustable proportioning valve, then it's up to your adjustments.
Tire size and w/c diameter also a consideration .

I'd do it, - with extra caution IF caught in the rain !