What was found
DURING TAXIING FOR DEPARTURE AND PERFORMING FUNCTIONAL CHECKS PER CHECK LIST, FLIGHT CREW RECEIVED FAILURE INDICATION FOR HYDRAULIC SYSTEM TEST. (NR 3 LT ENGINE DRIVEN PUMP) WOULD NOT DEPRESSURIZE. AIRCRAFT RETURNED TO GATE WITHOUT FURTHER INCIDENT. MAINTENANCE PERSONNEL FOUND CANNON PLUG TO NR 3 LT ENGINE DRIVEN PUMP DEPRESSURIZATION CANNON PLUG BAD. PART NOT AVAILABLE. DEFERRED PER MEL 29-11-1. (X)