Has the valve body been modified? Are all of the check balls, springs and spools back in correctly? Is it the correct valve body - C4/5 look much the same but aren't. Also, the 4 and 6 cylinder versions plus the Pinto version don't necessarily have the same valve body as the V8 version and that thin plate directing flow needs to be modified correctly if the tranny has a shift kit installed. Then - is the front pump(and the only one)cracked or heavily worn? Next and the easiest to check - the modulator needs to be connected to a full vacuum source (below the throttle plates) not the ported vacuum (above the throttle plates). Also, make sure the tubing to the modulator is in good shape, no leaks. There is an elbow at the modulator itself that likes to crack every few years or the steel tubing rusts.