What was found
DURING ENGINE RUNS FOR PHASE INSPECTION, LT ENGINE STARTED AND BOTH VACUUM FAIL BUTTONS PULLED ON GAUGE. VACUUM PRESSURE NORMAL. RT ENGINE WAS NOT RUNNING. RT ENGINE STARTED, ALL INDICATIONS NORMAL. LT ENGINE SHUT DOWN WITH RT ENGINE RUNNING AND BOTH BUTTONS REMAINED PULLED, BUT VACUUM FELL TO 1 INCH. TROUBLESHOOTING REVEALED CHECK VALVE DEFECTIVE IN VACUUM MANIFOLD. REMOVED AND REPLACED MANIFOLD, AIRBORNE TECHNICAL SERVICE INSTRUCTION CALLS FOR 1 YEAR INSPECTION AND 10 YEAR REPLACEMENT. THIS UNIT MANUFACTURED 10-75.