AN ACR LTT CREW WAS CHALLENGED BY AN ACR MAINT INSPECTOR ON THEIR KNOWLEDGE OF THEIR MEL PROCS.

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MY CAPT AND I FLEW 2 PAX CARRYING LEGS OF A TRIP WITHOUT COMPLYING WITH A REQUIRED MAINT MEL PROC. IT TOOK AN FAA LINE CHK BY A MAINT INSPECTOR TO POINT OUT TO US THAT WE HAD DONE SO; AND WERE ABOUT TO FLY A THIRD LEG WITHOUT DOING SO. WE HAD AN INOP MAIN SYS HYD PRESSURE GAUGE. WE OFTEN FLY SEVERAL DIFFERENT AIRPLANES EACH TRIP; MANY OF WHICH HAVE MEL WRITE-UPS. MOST OF THE MEL WRITE-UPS REALLY ARE SIMPLE; COMMON SENSE ITEMS THAT REQUIRE NO SPECIAL PROCS. WE ALWAYS CHK TO SEE WHAT IS WRITTEN UP; AND SHOULD ALWAYS READ THE MEL TO SEE IF THERE ARE ANY SPECIAL PROCS. IN THIS CASE; THE CAPT MENTIONED TO ME THAT THE MAIN SYS HYD PRESSURE GAUGE HAD A LITTLE BIT OF HYD FLUID LEAKING INTO IT. IT HAD BEEN WRITTEN UP BY THE LAST CREW AND I REALLY DIDN'T THINK MUCH OF IT; AND NEITHER DID HE. WE PROCEEDED WITH 2 LEGS OF OUR TRIP. AN FAA MAINT INSPECTOR RODE WITH US ON THE THIRD LEG; AND BEFORE WE STARTED; HE ASKED US ABOUT THE PROC REQUIRED BEFORE ENG START WITH THE GAUGE INOP; AND NEITHER OF US KNEW IT. WE PULLED OUT THE MEL; AND SURE ENOUGH; THERE IS AN IMPORTANT; REQUIRED PROC TO FOLLOW TO BE SURE THE HYD SYS WAS WORKING PROPERLY; AND THAT WE WOULD HAVE HYDS FOR THE FLT. IT WAS OBVIOUS TO THE INSPECTOR WE HAD NOT DONE THIS PREVIOUSLY. TO TOP THAT OFF; THE CAPT AND I FAILED TO LOCK OUR COCKPIT DOORS; WHICH IS A NEW PROC FOR US BEFORE TKOF. WE USED TO LOCK THEM AFTER TKOF; AND IT'S NOT ON OUR NEW CHKLIST.