Hanging brakes - help needed

[lutronjim]

Couldn't find old post so starting new.

C10 disk front/drum back with booster. Master cylinder and proportioning valve less than 3 years old via disk brake kit. Booster old from junk yard for power drums.

Quick stops - brakes wont fully release. Figured its triggered by high pressure.

Got wheels off ground. Engine off - I pushed on pedal as hard as I could. Three tries and no hanging front or back.

Cranked engine - pushed hard on pedal - front brakes hung - rears free.
Pulled vac hose from booster - fronts still hung. (One time on road before pulling hose released them.) Quick light pumping of pedal freed them.

Replaced vac hose - pushed hard on pedal - front brakes hung - rears free.
Quick pumping of pedal by hand freed them.

Just fronts hanging sounds like proportioning value. But why does it only hang when booster active?

Help would be greatly appreciated.