AN MD80 IN CRUISE AT FL330 HAD L HYD SYS PRESSURE AND QUANTITY DECREASE. MAINT ADVISED USE OF THE XFER PUMP. WHEN ON; R SYS PRESSURE WAS LOST.

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WE WERE ENRTE SAN TO ZZZ WHEN L HYD SYS INDICATOR STARTED TO VIBRATE THEN DECREASE TO LESS THAN 6 GALLONS. CAPT CALLED ZZZ1 MAINT FOR ADVICE AND WAS TOLD THAT DURING APCH; XFER PUMP SHOULD BE TURNED ON. HOWEVER; WE SHOULD TURN IT OFF IF L QUANTITY DECREASED FURTHER. DURING APCH; FLT ATTENDANTS COMPLAINED OF FUMES. CAPT REQUESTED ARFF TRUCKS TO MEET ACFT AS PRECAUTION. CAPT MADE PA TO PAX TELLING THEM TRUCKS WERE ONLY A PRECAUTION. ON DOWNWIND; AS XFER PUMP WAS TURNED ON AS PER COMPANY'S PREVIOUS INSTRUCTIONS; BOTH L AND R HYD SYS PRESSURE LOW LIGHTS ILLUMINATED AND STAYED ON STEADY. L QUANTITY HAD DROPPED; R REMAINED STEADY AT 13. FLAPS COULD NOT BE EXTENDED WITH THIS LACK OF HYD PRESSURE. CAPT TOLD ME TO TURN OFF XFER PUMP. AS XFER PUMP WAS TURNED OFF; R SYS PRESSURE WAS RECOVERED. WE EXTENDED GEAR AND FLAPS AND MADE A NORMAL LNDG. THIS COULD HAVE BECOME A NASTY SIT IF THE CAPT DIDN'T HAVE THE VISION OF FORETHOUGHT TO ASSESS AND PLAN OUT OUR COURSE OF ACTION. IT SEEMS THAT ALL PRESSURE WAS LOST DUE TO SEIZURE OF XFER PUMP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT AFTER A LATER REVIEW; THE PROCS MANUAL DID DEAL WITH THE SIT ENCOUNTERED AND DID NOT REQUIRE USE OF THE PWR XFER UNIT. THE RPTR SAID WITH THE PWR XFER UNIT OPERATING; THE R HYD SYS DROPPED TO TRIGGER THE LOW PRESSURE WARNING LIGHT. THE RPTR STATED THE CAPT USED THE IDEA THAT WHEN SOMETHING IS TURNED ON AND THE EFFECT IS NEGATIVE; THEN TURN IT OFF. THE RPTR SAID THIS WAS DONE AND R SYS PRESSURE RECOVERED.