B737-400. MAINT DEFERRED THE L MAIN FORWARD BOOST PUMP LOW PRESSURE LIGHT. ACTUALLY THE L MAIN FORWARD BOOST PUMP WAS MALFUNCTIONING.

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CAPT AND I TOOK ACFT FROM SFO TO CYVR ON SEP/FRI/04. IT WAS CARRYING AN MEL 28-3; IN WHICH THE L MAIN FORWARD BOOST PUMP PRESSURE LIGHT WAS DEFERRED; ASSUMING THE PUMP WAS STILL FUNCTIONAL. WE SPOKE TO THE PREVIOUS CREW WHO TOLD US THAT THE LIGHT ONLY WENT ON AT ALT IN CRUISE. WE THOUGHT IT STRANGE THAT A LIGHT FAILURE WOULD MANIFEST ITSELF IN SUCH A SPECIFIC PHASE OF FLT. LOOKING AT THE MAINT LOG; WE COULD SEE THAT THERE WAS AN ATTEMPT TO FIX THE LIGHT WITH A NEW LOW PRESSURE SWITCH. THIS FIX WASN'T SUCCESSFUL; AND THE AIRPLANE WAS RELEASED AGAIN UNDER THE SAME 28-3 MEL. THIS HISTORY ALSO CONTRIBUTED TO OUR DOUBT THAT THIS ACFT WAS PROPERLY RELEASED WITH THE PROPER MEL. CAPT AND I WERE CONCERNED THAT THE AIRPLANE MIGHT BE FLYING WITH A BAD PUMP; AND WITH THE KNOWN HAZARDS OF FAILING FUEL PUMPS; THIS WAS SOMETHING WE WANTED TO AVOID IF IT WAS TRUE IN THIS CASE. CAPT CALLED MAINT TO ASK HOW IT WAS KNOWN THAT THE PUMP WAS OK. THEY SAID ITS FUNCTION WAS CONFIRMED ON THE GND. THE PREVIOUS CREW SAID THE LIGHT ONLY CAME ON IN CRUISE. HOW WAS IT KNOWN THE PUMP WASN'T FAILING INFLT? WE COULDN'T KNOW SO WE PRESSED ON TO CYVR. SHORTLY AFTER LEVELOFF THE LOW PRESSURE LIGHT ILLUMINATED. CAPT SUSPECTED THE PUMP WAS NOT FUNCTIONING IN CRUISE; SO HE DECIDED TO VERIFY ITS FUNCTION. HE TURNED ON THE IGNITION SWITCHES; OPENED THE XFEED VALVE AND TURNED OFF THE L AFT BOOST PUMP; LEAVING THE FORWARD L PUMP SWITCH ON. HE LEFT THE PUMPS IN THIS CONDITION FOR JUST A FEW MINS; BUT IN THIS TIME A 300 LB IMBAL OCCURRED. I DIDN'T SEE THAT ANY FUEL WAS USED FROM THE L TANK AT ALL DURING THIS TIME. THIS LACK OF USE OF L TANK FUEL SEEMED TO CONFIRM THAT THE PUMP WASN'T WORKING. CONSULTING MAINT; WE WENT THROUGH THE CHKLIST FOR LOW PRESSURE LIGHT ILLUMINATION; WHICH RESULTED IN THE SUSPECT PUMP BEING TURNED OFF. WE COMPLETED THE FLT. FOLLOWING THE TRIP; CAPT TOLD ME THAT MAINT INDEED ENDED UP REPLACING THE PUMP. I FULLY UNDERSTAND THAT THE MAINT SYS AS IT EXISTS IS TO BE RESPECTED AND GENERALLY WORKS BEAUTIFULLY; AND THAT WE; THE CREW; ARE NOT TEST PLTS. BUT IN THIS CASE TO NOT DO ANYTHING WAS TO APPARENTLY FLY WITH A FAULTY FUEL PUMP; A POTENTIALLY DANGEROUS CONDITIONS; AS WAS EVIDENCED BY THE CHKLIST ACTION INSTRUCTING CREW TO TURN THE PUMP OFF. AS EVERY SIT IS UNIQUE; I FEEL CAPT USED HIS CAPT'S AUTH WISELY.