Proportioning valve

AJ/FormS said
A P-valve reduces the pressure to the rear brake cylinders to a percentage of whatever pressure is being sent to the front.

Not quite.
They do nothing when the line pressure is moderate.
They proportionally reduce the increase in line pressure above the set point.

AJ/FormS said
There is only one reason to run a valve like this, which is to prevent the rear brakes from locking up prior to the fronts

Yes its used with front disk-rear drum combinations to offset the amplification of braking force of the duo-servo drum system in the situations where heavy braking has shifted the weight to the front.

AJ/FormS said
When the rear locks up first, at speed, the rear usually steps out from behind the fronts and quickly sends you into an irrecoverable spin.

That pretty much sums it up.