A C152 ON TAXI HAD THE L BRAKE FAIL. ACFT FLOWN TO HOME BASE WHERE INSTRUCTOR PLT SVCED BRAKE FLUID AND BLED BRAKE LINES. NO LOG ENTRY.

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ON MAY/TUE/05; I WAS TAXIING WITH A STUDENT IN ZZZ1. AS WE APCHED THE DEP END OF THE RWY; MY STUDENT STARTED TO BRAKE IN ANTICIPATION OF THE L TURN ONTO THE RWY. AS SHE APPLIED THE BRAKES; THE ACFT STARTED TURNING TO THE R. I TOOK CTL; APPLIED FULL L RUDDER/BRAKE AND ENSURED THAT THE PWR WAS IDLE. THE ACFT CONTINUED TURNING TO THE R UNTIL IT CAME TO A FULL STOP; PERPENDICULAR TO THE TXWY; BUT STILL ON THE PAVED SURFACE. WE SHUT THE ENG DOWN AND INSPECTED THE L BRAKE SYS. THERE WAS NO PRESSURE IN THE BRAKE PEDAL; AND NO APPARENT LEAKS IN THE BRAKE LINES. WE MANUALLY TURNED THE ACFT TO FACE THE PROPER DIRECTION; AND DECIDED TO CANCEL THE LESSON AND RETURN TO OUR HOME BASE IN ZZZ2. THE REST OF THE FLT WAS UNEVENTFUL. UPON OUR ARR; I NOTIFIED THE OWNER/OPERATOR THAT THE L BRAKE WAS INOP. SINCE ANOTHER STUDENT WAS SCHEDULED TO FLY THE SAME ACFT; HE DECIDED THAT WE SHOULD FIX THE BRAKES ASAP. I WAS GIVEN A HYD FLUID DISPENSER AND INSTRUCTED ON HOW TO FILL THE L MASTER CYLINDER RESERVOIR. I FOLLOWED THE INSTRUCTIONS AND FILLED THE RESERVOIR; BUT THE BRAKE PEDAL DID NOT DEVELOP ANY PRESSURE. I WAS THEN TOLD THAT THE BRAKES PROBABLY HAD AIR IN THE LINES AND NEEDED TO BE BLED. I WAS GIVEN A DIFFERENT PUMP; AND WAS INSTRUCTED ON HOW TO USE THE PUMP TO BLEED THE BRAKE LINE. I ACCOMPLISHED THIS TASK; AND THIS SOLVED THE PROB. THE BRAKE DID HAVE PRESSURE AND WORKED NORMALLY. IN DOING THIS WORK; I WAS UNDER THE IMPRESSION THAT I WAS ACTING UNDER MY AUTH AS PLT UNDER 14 CFR 43.3(G) AND APPENDIX A TO PART 43(C). THE FORMER STATES; 'THE HOLDER OF A PLT CERTIFICATE...MAY PERFORM PREVENTIVE MAINT ON ANY ACFT OWNED OR OPERATED BY THAT PLT...' WHILE THE LATTER LISTS 'REPLENISHING HYD FLUID IN THE HYD RESERVOIR' AS 'PREVENTIVE MAINT.' WHAT I DIDN'T REALIZE IS THAT THERE IS A DIFFERENCE BTWN REPLENISHING HYD FLUID IN THE HYD RESERVOIR; AND BLEEDING THE BRAKE SYS. AT THE TIME; THE DIFFERENCE TO ME WAS SUBTLE: THE RESERVOIR BEING FILLED FROM THE BOTTOM RATHER THAN FROM THE TOP. BLEEDING BRAKES IS NOT CONSIDERED PREVENTIVE MAINT; AND MUST BE PERFORMED BY AN AUTH MECH. ADDITIONALLY; I FAILED TO RECORD THE MAINT THAT I PERFORMED IN THE ACFT'S MAINT RECORD; IN ACCORDANCE WITH 14 CFR 43.9.