Front brakes retaining pressure

[88Ironduke]

My 72' didn't come with power brakes so I added a booster and master cylinder to fit. I've used my stock 72' disc/drum proportioning valve, its pretty grungy looking but seems serviceable. New calipers/pads and fresh bearings have been installed. I changed the brake pedal input shaft to the shorter one for power brakes and everything seemed to be peachy. However I noticed that my truck got hard to push around. I thought the drum brakes were sticking/hanging up but it turns out both left and right front brakes were being applied. I tracked the pressure being held all the way thru the proportioning valve to the master cylinder. If I crack the line at the master cyl and let off a bit of pressure the brakes release as advertised. If the push rod is adjusted out to far will the master cylinder hold brake pressure on the front brakes and not the back?

Thoughts?
Paul