Help me understand the brake proportioning valve

I don't think it would in any way it would cause pulsing.

When the pressure in the brake lines goes above 300 psi, the pressure to the rear brakes is proportionally reduced.

Master, Front Disk, Rear Drum
250 psi 250 psi 250 psi
300 psi 300 psi 300 psi
400 psi 400 psi 350 psi
500 psi 500 psi 400 psi

The drum brakes are self-energizing so they don't need as much hydraulic pressure as th calipers. In addition the harder the braking the more weight shifts off the rear to the front. The proportioning valve helps prevent the rear drum brakes from locking up under hard braking.

https://www.web.imperialclub.info/Repair/Lit/Master/219/Page06.htm