As above check the vac line between the intake manifold and servo for splits etc. The one way valve is only there to prevent positive pressure entering the servo when on boost. However if this valve has stuck in the closed position then it will prevent vaccuum from reaching the servo.

The way to test the servo is - Engine off, pump the brake pedal a few times to depleat the vacuum from the servo (the brake pedal should now be solid if not there may be air in the hydraulic system somewhere). Hold your foot on the brake pedal and start the engine, the pedal should sink as vacuum builds up inside the servo.