ON A B737-300; WHILE HYD SVCING WAS TAKING PLACE; THE CREW COMPLETED THE BEFORE START CHKLIST AND THE PUMP WAS ACTIVATED. TECHNICIAN EXPRESSED CONCERN OF PUMP RUNNING WHILE SYS BEING SVCED. NO FLAG ON SWITCH.

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CALLED MAINT IN-RANGE FOR 'B' SYS HYD SVCING. AFTER PARKED AT GATE FOR 25 MINS (WE HAD ARRIVED 5 MINS EARLY); I HAD LEFT FOR FOOD. WE HAD A FLT ATTENDANT CHANGE; AND OTHER STANDARD DISTRS -- AND 5 MINS BEFORE PUSH TIME; I COMPLETED MY BEFORE START 'FLOW;' AND NOTICED THE 'B' SYS WAS NOW READING FULL. I ASKED THE FO IF MAINT HAD COMPLETED THE LOGBOOK FOR HYD FLUID SVCING. HE SAID HE HAD NOT SEEN A MECH. (FO HAD REMAINED IN THE COCKPIT AREA DURING GND TIME.) AS WE BEGAN THE BEFORE START CHKLIST; A MECH ARRIVED EXPRESSING CONCERN ABOUT OUR (MY) TURNING ON THE 'B' SYS PUMPS (DURING MY FLOW). HE STATED THAT THE SYS COULD HAVE BEEN BLED; OR PUMPS DAMAGED; OR FLUID SPILLED ON HIM AND THAT WE SHOULD HAVE COORDINATED WITH HIM PRIOR TO TURNING THE PUMPS ON. I APOLOGIZED (HE SAID NO INJURY OR DAMAGE HAD OCCURRED); BUT TOLD HIM I HAD NO IDEA THAT HE WAS STILL SVCING THE SYS. A CONVERSATION WITH A MECH LATER THAT DAY CONFIRMED MY EXPECTATION THAT SWITCHES SHOULD BE FLAGGED IF THEY COULD CAUSE DAMAGE OR INJURY IF MOVED DURING SVCING OR REPAIR. THIS IS THE SECOND SIMILAR EVENT I'VE BEEN INVOLVED WITH IN THE PAST 6 MONTHS. THE OTHER INVOLVED PACK SWITCHES DURING REPAIR (A MECH ANGRILY ARRIVED WITH A FLAG). WE HAVE NO WRITTEN GUIDANCE IN OUR FOM. YET APPARENTLY THERE IS GUIDANCE IN THE MAINT PROCS. THESE PROCS SHOULD BE ADHERED TO INSTEAD OF AFTER-THE-FACT ADMONISHING PLTS; OR WORSE; DAMAGE OR INJURY. I SPOKE WITH A CHIEF PLT. HE STATED HE WOULD PHONE MAINT REGARDING THEIR PROCS. I WOULD RECOMMEND OUR FOM ADDRESS SUCH PROCS.