proportioning valve questions

yes, see posts 2 and 4
And heres why:
Disc brakes require a lot more hydraulic pressure to do their job. Without the P-valve ,the rear hydraulics receive this same high pressure and they lock up.Edit; Come on too hard,and skid the tires.
The rear pressure has to be reduced, EDIT; to delay the rear system until the front is fully engaged.
If you still have 9 inch rear drums, you should probably get the aftermarket adjustable type. It will allow you to reduce the rear pressure by just the right amount.

EDIT; The rear system has to be delayed, and restricted,So that it comes on after the front is already working hard, and must not come on with enough power to skid the tires. If the rear skids uncontrollably, the car becomes very difficult if not impossible to steer, and likes to swap ends.
Remember when you were a kid, riding your 1-speed bicycle, with the brake in the rear hub.We would pedal as hard as we could and jam on that rear brake, and slide the rear around, doing awesome fishtails, all the while trying not to wipe out.
Well in a car thats kinda what happens except the front brakles are doing some 70% to 80% of the stopping, so the rear likes to get out sideways on the slightest of turns. And can put you backwards in a hurry.Unless al 4 tires are sliding. Then different physics take over.