The purpose of the proportioning valve is to engage the rear brakes before the front. This valve should be part of the MC on a dual system. I assume it would be beneficial on drum/drum systems. I don't know if the 60 GMC had a booster ( if I remember correctly, they were beginning to be installed in some vehicles about that time tho my 1960 Ford truck didn't have one) but I would guess you would want one.
