B737-900
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
General Maintenance Action
Aircraft; Procedure; Human Factors
During preflight I performed an Optional Check (per our Flight Manual) of the Standby Hydraulic System.  I found the Standby Pump indicating normal operation; but the Standby Low Pressure Light remained illuminated.  Maintenance was called and they confirmed the low pressure indication and began troubleshooting.  What we had was a pump running; but not putting out any pressure.  After various tests and about 45 minutes; our mechanics informed us they had fixed the problem.  The Maintenance Logbook [indicated that Maintenance] found the Hydraulic Supply line to Standby Pump partially disconnected at the Standby Reservoir.  Reconnected same. Verified proper operation of Standby Rudder and Alternate Flaps.  Verified Quick Disconnect at reservoir is secure; OK to continue. There is no way to know how long this aircraft had been flying without a Standby Hydraulic System.
Reporter stated the Optional Check of the Standby Hydraulic System use to be part of their routine Preflight Check procedures in their Flight Manual for the B737 aircraft. He felt he should check the system and that's when he noticed the Standby Pump indicated a normal operation; but the Standby Low Pressure Light remained illuminated. He does not recall the Standby Hydraulic Reservoir being low; especially since he did not see any evidence of hydraulic fluid leaking. He got the impression; when he spoke with company mechanics; that the partially disconnected hydraulic supply line to the Standby pump was a Quick Disconnect type coupling; which he suspects is probably why no hydraulic leak occurred; but also prevented any hydraulic fluid flow to the Standby Pump.
Captain reports finding the Standby Hydraulic System Pump indicating normal operation; but the Standby Low Pressure Light remained illuminated when he performed an Optional Preflight Check on a B737-900 aircraft. Maintenance found Hydraulic Supply Line to Standby Pump partially disconnected. Unknown how long aircraft had been flying without a functional Standby Hydraulic System.