All four brakes not releasing - advice needed

Ok guys, All four of my brakes release very slowly. All four wheels get hot due to extra drag. However they do release after some time as I can spin the wheels freely when they cool down. The car has new (rebuilt) Kelsey Hayes callipers, new rear drums and hardware, new distribution valve and new Master Cylinder.
My guess is that it's something to do with the brake fluid not relieving backward somehow after the pedal is released. Anyone have experience with this?