I assume front disc, an OEM set up with meter valve, and proportion valve beside distribution block? Is it the 3 piece or 2 piece setup? Did you flush and bleed MC before installation. Do you see the bleeds release fluid when in the return position. The rear drum should have a residual valve to hold some pressure on the rear drums, but if the Secondary MC piston is not uncovering the bleed there is no where for the pressure to release too. Both rear wheels show signs of dragging?

If both it would point to MC. The proportioning valve should not be causing this unless it is clogged up somehow and acting as a check valve versus pressure control. Easy to disassemble and clean, but should not be the issue. Especially if the brakes where not dragging before the change over.