Disc brakes locked

^^^^^^^This x2!

For the record, even the residual valves in the master (if it even still has them) only hold about ten psi. It's to keep the cups in the wheel cylinder expanded and keep air from being sucked in. At best it will only cause a light drag on a disc brake caliper of the size commonly used on our cars. It won't compound with every step of the pedal, it will only hold ten psi.

Proportioning valves are used because it requires more pressure to work a disc brake caliper than a drum brake wheel cylinder. The valve holds off pressure to the rear brakes for equal braking performance. They aren't needed in an all drum brake system.