[Bruce Edwards]

What kind of master are you using? One made for drum brakes will have built in check valves and cause the caliper to hang till the pressure bleeds off. Or does the car have any check valves in the lines? These would be small and look like long connectors holding to lines together.
Of course it you pinched a flex line it will do the same thing.
Also if the caliper was rebuilt and the o-ring was twisted it will also cause the caliper to hang.