A C172 CFI RPT ON THE TEMPORARY LOSS OF L BRAKING BY HIS STUDENT AND THE UNSATISFACTORY RESPONSE FROM THE FBO WITH REGARDS TO THE CAUSE OF THE FAILURE AT ZZZ; US.

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I WAS ACTING IN THE CAPACITY AS FLT INSTRUCTOR. I WAS IN THE PROCESS OF CHKING OUT A COMMERCIAL PLT WHO HAS INST AND MULTI-ENG RATINGS IN ADDITION TO HIS COMMERCIAL RATING. I HAD FLOWN WITH HIM BEFORE IN THE SAME MAKE AND MODEL ACFT. I HAD NOT SIGNED HIM OFF; HOWEVER; AT THIS POINT. WE TAXIED TO THE RUNUP AREA ADJACENT TO THE RWY PRIOR TO RELEASE. I HELD THE TOE BRAKES DURING THE RUNUP. EVERYTHING SEEMED OK. AFTER RUNUP THE PLT IN THE L-HAND SEAT CALLED THE TWR; ANNOUNCED READY FOR TKOF; AND HE RECEIVED CLRNC TO 'POS AND HOLD.' I HAD RELEASED THE BRAKES PREVIOUSLY AND ANNOUNCED; 'I HAVE RELEASED THE BRAKES.' AS THE L-HAND PLT TAXIED TOWARD THE RWY; HE SUDDENLY ANNOUNCED; 'I'VE LOST THE L BRAKES!' THE ACFT TURNED SHARPLY TO THE R. PWR WAS PULLED BACK AND BOTH OF US TRIED TO APPLY THE BRAKES. INSTINCTIVELY; I PUMPED THE BRAKES SEVERAL TIMES; BUT THE ACFT TAXIED OFF THE ASPHALT ANYWAY AND THE PROP STRUCK A TXWY LIGHT. THIS DID MINOR DAMAGE TO ONE BLADE. THE ENG WAS STILL RUNNING AND WE SHUT THE ENG DOWN. WE HAD THE ACFT TOWED BACK TO THE FBO FOR INSPECTION AND REPAIR. THE MECHS RPTED THAT THE BRAKES SEEMED TO BE WORKING OK NOW. PHOTOS WERE TAKEN THAT SHOWED SKID MARKS ON THE TXWY. THE R-HAND TIRE WAS SCUFFED. (THE SKID MARKS WERE ONLY OVER THE LAST 20 FT OF THE ASPHALT.) I DON'T KNOW; AT THIS TIME; WHY THE L-HAND BRAKE WENT OUT. I JUST KNOW THAT IT DID. I AM VERY CONCERNED THAT IT MAY GO OUT AGAIN. PERHAPS THE SAME ACFT WITH A STUDENT LNDG COULD RESULT IN A MUCH MORE SERIOUS INCIDENT. I AM ALSO CONCERNED THAT THE FBO IS NOT LOOKING INTO THE CAUSE OF THE LOSS OF THE BRAKE PROBLEM AND IS CONCENTRATING ON TRYING TO ASSESS BLAME ON THE L-HAND SEAT PLT AND MYSELF. I HAVE NUMEROUS HRS IN THAT MAKE AND MODEL AND THE L-HAND SEAT PLT HAS APPROX 600-700 HRS. YOU JUST DON'T SUDDENLY VEER R AND RUN OFF THE RWY FOR NO REASON IF YOU ARE AN EXPERIENCED PLT. THERE IS SOMETHING WRONG IN THE BRAKE SYS ON THIS ACFT.