1972 F750 brake booster

Dump truck. Chassis mount booster. Replaced 5 years ago with rebuilt (Cardid, I think, from NAPA). When I replaced it I also replaced vacuum lines and vacuum check valve (the original plastic check valve was junk-would sometimes stick open, blocking intake suction from reaching booster-with new valve this cannot happen).

Current problem: intermittent hard pedal (no boost), yet vacuum gauge reads correct (about 20+). Holds vacuum for days with engine not running, so obviously no vacuum...

1972 F750 brake booster