A B737-200 WAS DISPATCHED IN NON COMPLIANCE WITH A HYD LOW PRESSURE WARNING LIGHT AND AN ENG DRIVEN HYD PUMP DEPRESSURIZATION VALVE DEFERRED AS INOP IN CONFLICT WITH THE MEL.

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ACFT WAS CARRYING AN MEL 29-4 (INOP #1 SYS A LOW PRESSURE INDICATOR LIGHT). AFTER 2 LEGS; I NOTICED THAT THE SYS A PRESSURE INCREASED UPON STARTING ENG #1. CONSEQUENTLY; MEL 29-10 WAS ADDED. THE TEXT IN MEL 29-10 ONLY STATES NOT TO START THE ENG WITH THE TOWBAR CONNECTED. I DID NOT GO BACK AND REVIEW MEL 29-4; WHICH REQUIRES THE A-PUMP BE ABLE TO BE DEPRESSURIZED WITH THE SWITCH. IN OTHER WORDS; YOU ARE NOT ALLOWED TO HAVE BOTH MEL'S AT THE SAME TIME. THE MEL PAPERWORK WAS DONE BY OUR MAINT AND ALL I ASKED WAS IF THEY WERE ADDING THE 29-10 OR IF IT REPLACED THE 29-4. I WAS TOLD THAT THEY WERE ADDING THE 29-10 AND ALSO CARRYING THE 29-4. I THOUGHT BY REVIEWING THE RESTRS OF THE 29-4; BUT JUST DIDN'T DO IT SINCE OUR MAINT WAS HANDLING THE PAPERWORK. IF THE TEXT IN MEL 29-10 REFERRED TO 29-4 IN SOME WAY; THE MECH OR I WOULD HAVE CAUGHT IT. I FLEW THE AIRPLANE 1 LEG WHERE IT OVER-NIGHTED. THE PLT FLYING THE AIRPLANE THE NEXT DAY FLEW TO YYY. APPARENTLY MAINT CTL CAUGHT THE ERROR AND GNDED THE ACFT THERE. LESSON LEARNED: ALWAYS GO BACK AND READ THE LIMITATIONS ON EACH MEL IF YOU ADD ANOTHER TO MAKE SURE THEY ALL WORK TOGETHER. I FOUND OUT THAT I HAD FLOWN THE AIRPLANE WITH THIS ILLEGAL MEL BECAUSE I TOOK AN ACFT TODAY; JUN/SAT/03; FROM THE CAPT THAT GOT THE AIRPLANE FROM ME IN ON JUN/FRI/03. SUPPLEMENTAL INFO FROM ACN 586142: I RELEASED FLT WITH 2 MEL'S AND ONE OF THEM COULD NOT BE DEFERRED WITH THE OTHER. I DO WONDER WHY WE DON'T HAVE SOMETHING IN OUR SYS THAT WOULD FLAG CONFLICTING MEL'S OR AT LEAST GIVE YOU A POP-UP TYPE WARNING.