With 4 same sized tires on the car and front discs, yes the car needs to have the rear line pressure reduced to prevent pre-mature rear-wheel lock-up, and a possible resulting spin. As the rear tires gain footprint, the proportioning can be reduced. I run no proportioning at all with 295s on the back. and 15/16 wc's.
If you cannot find the stand-alone P-valve on your car, you can sub in the later style Combination valve. As it's name implies, it serves all three functions, namely distribution, proportion, and the brake failure warning light.
