You misunderstand the purpose of the proportioning valve, which is understandable because it's misnamed. It doesn't proportion pressure; it limits pressure to the back brakes.
Drum brakes work at much lower hydraulic pressure than disk brakes. The proportioning valve limits the maximum pressure to the back. Otherwise they would lock up while the fronts are just starting to work.
There might be a true proportioning valve out there, but I've never seen one. The common way to proportion front and rear hydraulic pressure it to use separate master cylinders actuated by a beam between them. The pedal linkage pushes on the beam, and the F/R ratio is adjusted by moving the linkage contact on the beam. It's standard race car stuff, and usually has a knob in the cockpit turning a screw that moves the contact point back and forth.
Drum brakes work at much lower hydraulic pressure than disk brakes. The proportioning valve limits the maximum pressure to the back. Otherwise they would lock up while the fronts are just starting to work.
There might be a true proportioning valve out there, but I've never seen one. The common way to proportion front and rear hydraulic pressure it to use separate master cylinders actuated by a beam between them. The pedal linkage pushes on the beam, and the F/R ratio is adjusted by moving the linkage contact on the beam. It's standard race car stuff, and usually has a knob in the cockpit turning a screw that moves the contact point back and forth.
