A B767 DURING REPLACEMENT OF A HYD COMPONENT HAD THE R RESERVOIR DEPLETED WHEN THE APU WAS OPERATED. CAUSED BY FAILURE TO PLACARD HYD AND PNEUMATIC CTLS.

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EARLY MORNING OUT OF ZZZ. ARRIVED AT ACFT FOR PREFLT. WENT OUTSIDE TO DO SAFETY INSPECTION. SAW MECHS WORKING UNDER R WING. SAID THEY WERE REPLACING HYD VALVE. TOLD ME NOT TO TURN ON ANY HYD PUMPS. I WENT BACK IN THE ACFT TO DO ORIGINATION PREFLT. THERE WAS NO SAFETY PLACARD DISPLAYED IN THE COCKPIT WARNING OF USAGE OF HYD PUMPS; SYS; ETC; AS IS USUALLY DISPLAYED. I FOUND THE MAINT SAFETY PLACARDS AND TURNED TO THE ONE WARNING AGAINST TURNING ON ANY HYD PUMPS AND PLACED IT ON THE FORWARD PEDESTAL. THE ACFT WAS STUFFY AND WARM IN THE COCKPIT AND CABIN. EXTERNAL PWR WAS CONNECTED BUT NO AIR WAS HOOKED UP. FLT ATTENDANT ASKED FOR SOME AIR. I STARTED THE APU AND TURNED ON THE PACKS; KNOWING THAT THE HYD RESERVOIRS WOULD BE PRESSURIZED; BUT I DID NOT THINK THIS WOULD PUT ANY PRESSURE OUT IN THE LINES. I WAS WRONG. VERY SOON THEREAFTER; I NOTICED THAT THE STATUS PAGE ON THE EICAS SHOWED THAT THE R RESERVOIR WAS AT ZERO (PRIOR TO STARTING THE APU) AND TURNING ON THE PACKS IT REACHED .60. ABOUT THE SAME TIME; ONE OF THE MECHS CAME IN AND SAID TO TURN OFF THE APU; AS I HAD PRESSURIZED THE RESERVOIR AND WE HAD LOST THE FLUID THAT WAS IN THE R SYS. I SHUT DOWN THE APU AND ASKED WHY HE DID NOT TELL ME NOT TO START THE APU. HE SAID THEY JUST FORGOT TO TELL ME. I ASKED IF ANYONE WAS HURT. HE SAID NO. I THEN PREPARED ANOTHER PLACARD TO DISPLAY WARNING NOT TO START THE APU IN CASE THE CAPT CAME IN WHILE I WAS OUT FOR WALKAROUND. OUTSIDE; I AGAIN APOLOGIZED AND ENSURED NO ONE WAS HURT. EVERYTHING WAS OK. THIS FAILURE TO COMMUNICATE COULD HAVE CAUSED A REAL PROB.