Is the brake light on the dash illuminated?
Inside the proportioning/combination valve there is a spool valve that moves to one side if the brake pedal is depressed while there is no pressure in one of the brake channels (Front or rear), such as what could happen if one side developed a leak... OR if you happened to push the pedal to the floor while bleeding the brakes... There would be pressure in te front, but not in the back, so the valve moved toward the back.... Usually this illuminates the brake warning light.
Sometimes the valve is designed to block further flow to the channel with no pressure... in effort to stop the loss of brake fluid. The valve is SUPPOSE to reset itself, but I have heard they sometimes dont and need to be reset manually.... Like KillerB said. Look for the button or nub in the front side of the combination valve and try to pop it back out.