I recently installed rear disc brakes to go with front disc brakes I put on a couple years ago. I installed a proportioning valve for the front disc/rear drums. The car stops, but the pedal is extremely low for four wheel discs. I don't think I have air in the line. Could be a vacuum issue and the power booster isn't operating properly or it could be that I need a front disc/rear disc proportioning valve. Has anyone ever had this issue?