A B767-300 AT GATE WITH MAINT WORKING A HYD LEAK THE WAITING CREW RPTED A WARM CABIN. STARTED APU WHICH PRESSURIZED HYD RESERVOIRS. CREATED PROB WITH TECHNICIANS WORKING LEAK.

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WHEN FO AND I GOT TO THE COCKPIT; THERE WAS A PLACARD ON THE PEDESTAL; SAYING MECHS WERE WORKING ON HYDS AND NOT TO OPERATE THE HYD SYS. FO CALLED MAINT TWICE TO SEE WHAT WAS GOING ON. THEY SAID THEY WERE WORKING ON LEAK ON THE R SIDE. IT WAS ALSO VERY HOT ON THE AIRPLANE (88-90 DEGS) AS THERE WAS NO EXTERNAL AIR CONNECTED. I CALLED RAMP TWICE REQUESTING AIR; TO NO AVAIL. I CALLED THEM ONE MORE TIME TO SAY THAT WE WOULD BE STARTING THE APU SHORTLY; IF WE DIDN'T GET THE EXTERNAL. WE DIDN'T; SO I STARTED THE APU (IT WAS NOW BOARDING TIME). A FEW MINS LATER; A MECH CAME RUSHING UP ASKING WHY WE HAD STARTED THE APU WHILE SOMEBODY WAS WORKING ON THE HYD LEAK. EVENTUALLY A LEAD MECH AND SUPVR CAME UP ASKING US THE SAME THING. EVIDENTLY THE APU HAD PRESSURIZED THE HYD SYS; CAUSING ALL HYD FLUID OF THE CTR SYS TO SQUIRT OUT. WE REPLIED THAT THE PLACARD SAID NOT TO OPERATE THE HYD SYS; BUT DIDN'T SAY ANYTHING ABOUT THE APU. THE SUPVR MADE IT SOUND LIKE A VERY SERIOUS EVENT; SO MY QUESTION TO HIM WAS; THAT IF THIS WAS SO SERIOUS; WHY THEN WAS THERE NO NOTE SAYING NOT TO OPERATE THE APU AND WHY DID NOBODY FROM EITHER RAMP OR MAINT SAY ANYTHING WHEN IT WAS CLR FOR ABOUT 10-15 MINS (FROM THE NUMEROUS TIMES WE CALLED THEM) THAT WE WERE ABOUT TO START THE APU? LEAK WAS EVENTUALLY FIXED AND THE REST OF THE FLT WAS UNEVENTFUL.