Did you replace the rear flexible brake hose?
You might also want to remove the inlet line at the valve and see if flow is available there. If it is, it is a bad valve.
From Master Power Brakes Website;
A tripped combination valve.
If you lose half your braking system and the combination shuttle valve cuts off half the system this can give you a hard pedal. This type of combination valve can be tripped during the bleeding process. To center the valve open a bleeder screw on the half of the system that still has pressure. Depress the brake pedal and tighten the bleeder screw. This should center the valve.