A B757-200 FLT CREW TAXIING FOR TAKEOFF NOTED AN EICAS MESSAGE R HYD QTY. THEY FOLLOWED CHECKLIST; COORDINATED WITH MAINT; AND WERE TOWED BACK TO GATE.

Narrative

DURING TAXI OUT FROM GATE; WE WERE GIVEN INSTRUCTIONS BY GND CTL TO TAXI TO RWY XXR VIA X HOLD SHORT XY FOR A 20 MIN WHEELS-UP TIME TO ZZZ1. ONCE THE BRAKES WERE SET AT XY; I DECIDED TO DO THE FLT CTL CHK. AS I PULLED UP THE LOWER EICAS SCREEN TO DISPLAY THE FLT CTLS; THAT WAS THE SAME TIME WE OBSERVED THE R HYD QUANTITY DISPLAY AT .24 AND THE PF SYMBOL NEXT TO IT. WE THEN GOT THE R HYD QUANTITY AND R HYD RESERVOIR EICAS MESSAGE. I DIRECTED MY FO TO CONTACT MAINT CTL AND I RAN THE APPROPRIATE CHKLIST. THE B757 HYD SYS PRESSURE-R (ONLY). THIS DIRECTED US TO SHUTOFF BOTH THE ENG AND ELECTRIC HYD PUMPS FOR THAT SIDE. I MADE A DECISION THAT WE SHOULD BE TOWED BACK TO THE GATE AS WE NOW LOST OUR NORMAL BRAKES (R HYD SYS) AND WE WERE NOW ON THE ALTERNATE BRAKES (L HYD SYS). APU WAS STARTED AND WE SHUT DOWN THE ENGS. GND CTL WAS INFORMED OF THE PROB. GND CTL SENT THE DEPT OF AVIATION OUT TO INSPECT THE ACFT. THEY CONFIRMED A FLUID LEAK AND GND CTL DISPATCHED THE FIRE DEPT. MAINT ARRIVED WITH A SPILL KIT AND BEGAN A CLEAN-UP. WE WERE TOWED BACK TO OUR ORIGINAL GATE WHERE MAINT STARTED THEIR INSPECTION. IT WAS FOUND THAT AN ANTISKID VALVE WAS CRACKED. A NEW ONE WAS INSTALLED AND THE SYS REPLENISHED; TESTED; AND RELEASED FOR FLT. NO OTHER PROBS WERE NOTED AND OUR FLT WAS COMPLETED TO ZZZ1.