A SOMEWHAT MUTED DISAGREEMENT BTWN THE FO AND PIC RELATED TO THE LEGALITY OF THEIR MEL'ED 29 PT 9 ITEM RELATED TO A SUPPOSED MALFUNCTION OF THEIR STANDBY HYD LOW QUANTITY WARNING SYS ON THEIR B737-700 IN SAN; CA.

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SCHEDULED PLANE CHANGE IN SAN. LAST LEG OF THE NIGHT. SECOND LEG AS A CREW MEMBER. AFTER PUSHBACK AND RECALL PRESS TO TEST; WE NOTICE A 'STANDBY HYD LOW QUANTITY' LIGHT ILLUMINATED ON THE OVERHEAD PANEL. RUN QRH AND AMONG OTHER THINGS IT SAYS; 'NON DEFERRABLE /CONTACT DISPATCH.' WE GO BACK TO THE GATE AND THE CAPT CALLS DISPATCH/MAINT. THE RESULT OF THAT CONVERSATION IS TO AMEND RELEASE WITH MEL 29.9 AND GO. I QUESTION THAT DECISION ON THE BASIS THAT I BELIEVE THE LIGHT IS OPERATING AS DESIGNED AND THE QRH SAYS NON DEFERRABLE. THE CAPT EXPLAINS THE RELATIONSHIP BTWN THE 'STANDBY' AND 'HYD B' SYS AND HE; DISPATCH AND MAINT BELIEVE WITH GOOD 'HYD B' QUANTITY WE CAN GO. I AM NOT ADAMANT IN MY POS; SO WE PUSH. (WE LOOKED FOR A CIRCUIT BREAKER; BUT COULD NOT FIND ONE.) WE DISCUSS THE SIT ALL THE WAY TO SJC (AND ALL THE NEXT DAY). WE AGREE THE CHIEF PLT ON CALL SHOULD HAVE BEEN BROUGHT INTO THAT INITIAL PHONE CONVERSATION. WE DECIDE TO CALL HIM IN SJC; IN THE EVENT WE HAVE ERRED AND A BROKEN AIRPLANE NOW SITS AT THE GATE. THE RESULTS OF THAT CONVERSATION LEAD TO THIS ASAP RPT. THE NEXT DAY; I FELT WE STILL HAD MADE THE WRONG DECISION -- AND FELT WORSE BECAUSE I WAS NOT MORE FORCEFUL IN MY OPINION. I THINK THE LIGHT WAS WORKING PROPERLY AND FOR SOME REASON THAT WAS NOT OBVIOUS TO EITHER OF US IN THE COCKPIT (NOR TO THE MAINT MAN 1000 MI AWAY); WAS TELLING US THAT WE HAD 'A PROB' WITH THAT PARTICULAR SYS. LESSON LEARNED; IF THERE IS A DIFFERENCE IN OPINION BTWN THE COCKPIT CREW; BREAK THE TIE WITH A VOTE FROM THE CHIEF PLT ON CALL. HIS INTEREST ARE MORE ALIGNED WITH THE FLT CREW THAN THOSE OF EITHER MAINT OR DISPATCH. SOUNDS HARSH; BUT I AM FINDING IT TO BE A REALITY. OTHER CONTRIBUTING FACTORS: LAST LEG OF THE NIGHT; FULL ACFT. WOULD HAVE HAD TO SWAP ACFT AND RELOAD ALL THE PEOPLE RESULTING IN AN HR DELAY. IT PROBABLY WEIGHTED ON ALL OUR MINDS. SUPPLEMENTAL INFO FROM ACN 619589: MAINT REFERRED TO MEL 29-9 STANDBY HYD LOW QUANTITY INDICATOR; 1 INSTALLED/0-REQUIRED. COMPLIED WITH ALL MEL PROVISIONS AND ADDITIONALLY WENT INTO WHEELWELL TO CONFIRM THAT HYD QUANTITY OF 98% MATCHED COCKPIT GAUGES AND TO CHK FOR LEAKS. THE FO LOOKED SKEPTICAL; BUT DID NOT OBJECT. I FEEL WE COMPLIED WITH THE MEL; WE WERE NOT RUSHED AND WOULD HAVE FELT COMFORTABLE WITH AN ACFT SWAP IF NECESSARY. ARR SJC; I CALLED DISPATCH AND ON CALL CHIEF PLT. CHIEF FELT WE SHOULD NOT HAVE FLOWN THE ACFT SINCE THE QRH STATES IT IS NOT DEFERRABLE. I CALLED MY ASSISTANT CHIEF PLT AM THE NEXT DAY. HE HAD A QRH AND MEL IN FRONT OF HIM AND FELT WE WERE LEGAL TO FLY. I AM CONFIDENT; BUT CONFUSED. PERHAPS A NOTE IN THE QRH UNDER STANDBY HYD LOW QUANTITY LIGHT THAT REFERS TO MEL 29-9 AND SAYING IT IS NORMALLY NOT DEFERRED OR IT CAN BE DEFERRED. WHEN THE MEL SAYS IT IS DEFERRABLE IF THE SYS IS INOPERABLE; BUT THE LIGHT IS ILLUMINATED; HOW DOES ONE KNOW IF THE SYS IS INACCURATE OR INOPERABLE? IF A SYS IS INOPERABLE; WHY IS THE WARNING LIGHT ILLUMINATED? I FELT THAT THE QUANTITY INDICATOR WAS NOT WORKING; BUT THEN HOW DID I REALLY KNOW?