problem bleeding back brakes

Take the line to the rear brakes loose at the proportioning valve and see if flow freely at that point. If not, it's in the valve, if it flows good, there is a downstream clog somewhere. If the valve isn't badly stuck, sometimes you can free it up by taking the plug out of the end of the housing and tapping on the spool valve inside. If it were me, I'd pull the valve off the car, remove the spool valve, and clean everything up real good before reassembly. don't get carried away sanding the spool or bore as this will ruin the valve. Just enough to get the spool to move freely. That spray brake cleaner works good on deposits that may be inside.