[Steve Shauger]

The pressure regulator valve under the master had two functions. It delays the engagement of the front disc, so the car did not nose dive when brakes were applied and also provided 30-40 psi to the rear wheel cylinders. The ensured the rear drum shoes engaged prior to the front calipers.This applies to front disc, rear drum configuration.

Updated:
From CRG/ John Hinckley Brake valve report
Rear brake line pressure regulator mounted on subframe function:
"The valve limits the amount of hydraulic pressure, at a controlled rate, to the rear wheels in proportion to the amount of pressure to the front wheels. This action prevents the rear brakes from locking up before the full effective braking effort is produced by the front disc brakes."