A MAULE M7 PILOT REPORTS THAT A LOCKED BRAKE ON A RWY LNDG CAUSED THE ACFT TO DEPART THE RWY.

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I WAS FLYING A MAULE M7-235 AMPHIBIAN ON BAUMANN FLOATS. THE ACFT OWNER AND CFI WAS THE R SEAT PAX. I RECEIVED A SINGLE ENG SEAPLANE RATING IN THIS ACFT IN DEC/06. THIS WAS MY FIRST FLT IN THIS ACFT SINCE MY FAA CHK RIDE. I WAS LNDG AT ZZZ; RWY 17 AFTER MAKING 7 WATER LNDGS AT A NEARBY LAKE. THE SKY WAS CLR WITH THE WIND 170 DEGS AT 8 KTS. THE MAIN GEAR BRAKES APPEARED TO BE 'ON' AS SOON AS THE MAINS TOUCHED DOWN; WITH THE L WHEEL BRAKE LOCKED. THE ACFT VEERED TO THE L AND DEPARTED THE RWY. FULL R RUDDER WAS APPLIED TO NO AVAIL. THE INSTRUCTOR RPTED THAT THE BRAKES ON THIS ACFT ARE 'VERY SENSITIVE.' THERE WAS NO DAMAGE TO THE ACFT OR TO THE PLT OR FLT INSTRUCTOR PAX. THE INSTRUCTOR AND I BELIEVE THERE ARE 2 POSSIBLE CAUSES TO THE ACFT LEAVING THE RWY. 1) I COULD HAVE APPLIED PRESSURE TO THE L RUDDER; ENGAGING THE BRAKE AND ULTIMATELY CAUSING THE BRAKES ON THE L MAIN TO LOCK. 2) ON THIS MODEL OF MAULE; THE PARKING BRAKE IS ENGAGED BY PULLING A TAB ON THE R SIDE OF THE INST PANEL. THE TAB IS COVERED WITH A YELLOW STICKER; BUT IS LOCATED TO THE L OF A SIMILAR LOOKING TAB WHICH IS THE CARB HEAT CTL. IT IS POSSIBLE THAT I INADVERTENTLY PULLED OUT THE PARKING BRAKE TAB INSTEAD OF THE CARB HEAT ON FINAL DURING PREVIOUS LNDGS AT THE LAKE. THE INSTRUCTOR TOLD ME THAT ONCE THE PARKING BRAKE TAB IS PULLED OUT; IT MUST THEN BE PUSHED IN; AND THE BRAKE PEDALS FULLY DEPRESSED TO DISENGAGE THE PARKING BRAKE. GIVEN THAT I WAS DOING WATER LNDGS; THERE WOULD NOT HAVE BEEN ANY NEED TO DEPRESS THE BRAKE PEDALS IF I HAD INADVERTENTLY PULLED OUT THE PARKING TAB TO DISENGAGE THE PARKING BRAKE. AFTER DISCUSSING THE SITUATION WITH THE FLT INSTRUCTOR; WE CONCLUDED THAT IN THE FUTURE I SHOULD TAKE 2 ACTIONS. 1) BE CERTAIN THAT MY HEELS ARE ON THE FLOOR OF THE ACFT WHEN LNDG. 2) TAKE EXTRA CARE IN PULLING THE CARB HEAT TAB ON FINAL. IN RETROSPECT; THE INSTRUCTOR TOLD ME NEVER TO ENGAGE THE PARKING BRAKES ON THIS AIRPLANE DURING MY SEAPLANE TRAINING; AND I WASN'T REALLY CERTAIN HOW TO ENGAGE THE PARKING BRAKE ON THIS TYPE ACFT. IT WOULD HAVE BEEN A GOOD IDEA TO SIT IN THE ACFT AND REVIEWED THE INSTRUMENTATION AND CTLS BEFORE FLYING THE ACFT AFTER NOT FLYING THIS TYPE FOR 4 MONTHS.