A DUCHESS BE76 PLT ROLLED INTO THE MUD AT THE END OF PAO'S RWY. ACFT EQUIP PROB L BRAKE FAILURE.

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I WAS ASKED TO PERFORM A ROUTINE FERRY FLT FROM SQL TO PAO. THE DUCHESS HAD BEEN SQUAWKED FOR THE L MAIN BRAKE NEEDING TO BE PUMPED. FBO MAINT HAD INSPECTED AND BLED THE BRAKES. I PERFORMED A PREFLT AND PAID SPECIAL ATTN TO THE L MAIN BRAKE TO INSPECT IF FOR ANY IRREGULARITIES. I FOUND NOTHING WRONG DURING THE PREFLT. I PERFORMED SEVERAL BRAKE CHKS DURING THE TAXI. DURING THESE CHKS; THE BRAKING SYS FUNCTIONED NORMALLY. THE FLT WENT NORMALLY WITH ALL SYS FULLY OPERATIONAL. THE RWY AT PAO WAS WET FROM RAIN; SO I DECIDED TO RESERVE BRAKING UNTIL THE ACFT'S AIRSPD WAS UNDER 15 KTS AND I USED FULL AERODYNAMIC BRAKING TO SLOW THE ACFT AFTER LNDG. DURING THE LAST 800 FT OF PAO RWY; I STARTED TO USE BRAKES SLIGHTLY TO SLOW THE ACFT AND I DECIDED TO USE THE LAST TXWY TO CLR THE RWY. UPON REACHING THE LAST TXWY; I APPLIED L BRAKE TO TURN OFF THE RWY BUT THE BRAKE BEGAN TO FAIL DURING THE TURN. I TRIED SEVERAL TIMES TO PUMP THE BRAKES DURING THE L TURN; BUT THE L BRAKE PEDAL WENT ALL THE WAY FORWARD WITH NO BRAKING PRESSURE. THE ACFT DID NOT HAVE ENOUGH TURNING RADIUS TO STAY ON THE LAST TXWY AND THE ACFT ROLLED INTO A MUDDY AREA AT APPROX 5 KTS; THEN SLOWED TO A STOP IN THE MUD. TWR ASKED IF I NEEDED ASSISTANCE AND I INFORMED THEM THAT I HAD A BRAKE FAILURE AND WOULD NEED TO BE TOWED BACK TO PARKING. ARPT TRUCK TOWED THE DUCHESS BACK TO ITS TIEDOWN. THERE WAS NO DAMAGE TO THE ACFT NOR TO THE ARPT. SINCE THE ACFT WAS ABLE TO CLR THE RWY AND TXWY; THE ARPT WAS ABLE TO REMAIN OPEN.