problem bleeding back brakes

still having problems with the 70 duster. installed new disc brake proportioning valve. rebuilt kelsey hayes 4 piston calipers. the problem I had was low brake pedal, so installed new rear wheel cylinders. Cannot get fluid to the back wheel cylinders. fluid does not seem to be going through the proportioning valve. I have been bleeding the brakes with vacuum bleeder, but no fluid. Car was originally drum brakes. Any ideas on what the problem is.
thanks wayne