FLC OF A B727 ABORTED TKOF DUE TO A LOSS OF HYD QUANTITY RESULTING IN BEING DIRECTED OFF THE RWY TO THE L AND THEN TO THE R. THIS TOOK THEM TO THE R RWY WHICH WAS CLOSED.

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ACR RECEIVED CLRNC FOR TKOF RWY 15L AT IAH. ON APPLICATION OF PWR; HYD SYS A QUANTITY GAUGE DROPPED TO ZERO. NO DROP IN SYS PRESSURE OR ASSOCIATED VISUAL ALERTING (LIGHTS). WE REJECTED TKOF CLRNC AND TWR INSTRUCTED US TO MAKE A R TURN OFF RWY ONTO TXWY WD. THIS WAS SECOND TAXI INSTRUCTIONS -- FIRST INSTRUCTION WAS TO TURN L OFF RWY TO TXWY WD THEN GET IN LINE. VERY CONGESTED TFC. AS WE WERE GIVEN SEVERAL TAXI INSTRUCTIONS; I FAILED TO CONFIRM EXACT TAXI CLRNC. CREW UNDERSTOOD TWR TO INSTRUCT US TO TURN R ON TXWY WD FROM RWY 15L THEN R TURN ON RWY 15R; WHICH WAS NOTAMED CLOSED. WE WERE AWARE THE RWY WAS CLOSED (RWY 15R) BUT FELT COMFORTABLE USING IT TO BACK TAXI FOR POSSIBLE DEP AFTER CLRING UP OUR ABNORMAL. WE WOULD NOT HAVE BEEN SO COMFORTABLE USING THE WESTERN TXWY FOR TAXI (TXWY WC) DUE TO IT BEING NARROW AND THE TURN REQUIRED FROM TXWY WD ONTO TXWY WC. A B727 NEEDS MORE ROOM TO TURN 90 DEGS. WE WERE INSTRUCTED TO STOP; AS WE WERE ON CLOSED RWY. WE TAXIED ABOUT 50 YARDS DOWN RWY 15R TOWARD THE N. TWR SENT AN ARPT VEHICLE TO ESCORT US AROUND RWY PATCH (REPAIR). LIGHTS WERE ON -- RWY 15R -- AND NO MARKERS OUT TO SHOW RWY UNUSABLE AS BACK TAXI. WE CLRED UP ABNORMAL AND ULTIMATELY DEPARTED. NOTHING WAS SAID FROM TWR OR US ABOUT THE BACK TAXI; HOWEVER; I INTEND TO ALWAYS CLARIFY MY TAXI INSTRUCTIONS; NO MATTER HOW BUSY ATC IS OR WHETHER THEY ARE GIVEN BY TWR OR GND CTL.