COMMUTER LTT HAD A HYD FAILURE PRIOR TO TKOF THEN MISSED THE CLRNC TAXIWAY RETURNING TO THE GATE.

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JUST AS I TAXIED ONTO RWY I NOTICED THE MAIN HYD PRESSURE FALLING BELOW MIN OPERATING PRESSURE (DRIVE SHAFT HAD SHEARED). THE FO NOTIFIED THE TWR THAT WE HAD TO RETURN TO THE GATE. THE TWR CTLR TOLD US TO CLR THE RWY AT TAXIWAY ALPHA; AND TURNED OFF THE RWY AT CHARLIE. I HAD TO USE FULL REVERSE PWR TO PREVENT THE PLANE FROM GOING INTO THE GRASS AT THIS INTXN. WE HAD TO STOP FOR SEVERAL SECONDS WHILE THE FO PUMPED UP SOME MORE HYD PRESSURE TO GIVE ME LIMITED NOSE WHEEL STEERING AND BRAKING. WHILE THIS WAS GOING ON THE TWR HAD TO HAVE A FLT GAR. WHEN WE GOT CLR OF THE RWY THE CTLR CHEWED ON US FOR AWHILE. THE FO APOLOGIZED PROFUSELY TO THE CTLR. WE THEN PROCEEDED TO THE GATE WITHOUT FURTHER ADO. IN RETROSPECT SEVERAL THINGS SHOULD OR COULD HAVE BEEN HANDLED DIFFERENTLY. I SHOULD HAVE HAD THE FO ASK THE CTLR WHERE TAXIWAY ALPHA WAS; OR I COULD HAVE STOPPED THE PLANE AND DUG OUT THE ARPT DIAGRAM CHART BEFORE WE STARTED UP THE RWY LOOKING FOR TAXIWAY ALPHA. I SHOULD HAVE BEEN MORE INQUISITIVE WHEN I SAW TAXIWAY BRAVO. WE BOTH THOUGHT ALPHA WAS LOCATED IN FRONT OF THE GATE. WHEN WE PASSED ALPHA THE CTLR SHOULD HAVE TOLD US TO TURN L AT THE TAXIWAY. THE TAXIWAY SIGN FOR ALPHA IS LETTERED ONLY ON THE S SIDE! IF IT WAS PROPERLY MARKED I WOULD NOT HAVE MISSED IT.