A C172 WHILE TAXIING DECLARED AN EMER AND MADE A RWY INCURSION DUE TO BRAKE FAILURE CAUSED BY TAXIING WITH THE PARKING BRAKE PARTIALLY SET.

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I WAS TAXIING FROM THE HANGAR FOR TKOF ON RWY 5R -- A DISTANCE OF APPROX 3 MI. I HAD JUST PICKED THE CESSNA 172 UP FROM MAINT; IN WHICH THE BRAKES HAD BEEN CHANGED. I WAS CLRED TO TAXI TO RWY 5R VIA TXWYS G; M; J; P; AND C. I NOTICED NO ABNORMALITIES DURING MY TAXI. I WAS OBSERVING FOR ANYTHING OUT OF THE ORDINARY DUE TO THE ACFT JUST COMING OUT OF MAINT. AFTER TURNING SBOUND ON TXWY C AND CONTINUING ABOUT 2/3 OF THE APPROX 2 MI; I DID A MAGNETO CHK WHICH REQUIRED USING THE BRAKES. THIS WAS DONE IN A NORMAL MANNER AND NO ABNORMALITIES WERE NOTED. I CONTINUED TO TAXI NORMALLY. AS I BEGAN TO TURN L ONTO TXWY C9 (LAST INTXN TO THE RWY); I NOTICED THAT I HAD NO BRAKE PRESSURE ON MY L PEDAL; AT WHICH TIME I DECIDED TO RETURN TO THE HANGAR. I NOTED THAT I HAD SOME BRAKE PRESSURE ON THE R PEDAL. HOWEVER; AS SOON AS I APPLIED PRESSURE TO THAT PEDAL IT FAILED. I THEN EMPLOYED FULL L PEDAL AND WAS ABLE TO NEGOTIATE THE TURN ONTO TXWY C9. I STRAIGHTENED THE ACFT OUT AND REALIZED THAT MY MOMENTUM WOULD CARRY ME ONTO THE APCH END OF RWY 5R. I ATTEMPTED TO CONTACT GND CTL 2-3 TIMES; BUT THEY ANSWERED OTHER TFC; AT WHICH TIME I TOLD THEM I HAD AN EMER. THEY THEN ANSWERED ME AND I CLRED THEM OF MY SIT. AN ACFT ON SHORT FINAL WAS GIVEN A GAR. I THEN SHUT THE ENG OFF; HOWEVER; IT CONTINUED ON; COMING TO REST ON THE L EDGE OF THE RWY. I ADVISED GND CTL THAT I COULD RESTART AND CONTINUE ACROSS THE RWY AND STOP ON THE OPPOSITE TXWY. THEY ADVISED FOR ME TO DO THAT. I RESTARTED AND WHEN I HAD ENOUGH MOMENTUM TO CLR THE RWY BY A SAFE DISTANCE; I SHUT THE ENG OFF AGAIN. I REQUESTED GND TO CONTACT MY HANGAR TO ARRANGE A TOW. DURING THE TIME I WAS DEALING WITH THE ATTEMPT TO MAKE THE L TURN OFF TXWY C; I REACHED FOR THE PARKING BRAKE TO SEE IF THAT WOULD PROVIDE ANY BRAKES. I NOTICED AT THAT TIME THAT THE PARKING BRAKE WAS PARTIALLY ENGAGED.