AN EXTRA EA300 HAD A R BRAKE O-RING SEAL REPLACED FOR LEAKING. BRAKE GND TESTED OK. ON FIRST LNDG; R BRAKE FAILED AND ACFT DEPARTED RWY. NO DAMAGE INCURRED.

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R BRAKE; NEW O-RING DUE TO LEAK. TESTED ON WAY TO RUN-UP AREA -- OK. IT WORKED OK BRIEFLY ON 2 TOUCH-AND-GOES. PRACTICED AEROBATICS FOR 40 MINS. ON LNDG; AFTER ABOUT 2/3 OF RUN-OUT; ON FIRST BRAKE APPLICATIONS; IT FAILED. ACFT SWERVED L ABOUT 20 DEGS. REPEATED ATTEMPTS TO GET EVEN A LITTLE R BRAKE FAILED. ACFT EXITED RWY AT ABOUT 15 MPH AND GROUNDLOOPED ABOUT 45 DEGS. NO DAMAGE TO ACFT OR ANYTHING ELSE. BRAKE REGAINED A VERY SLIGHT FUNCTION; AND I WAS ABLE TO TAXI VERY SLOWLY TO PARKING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE LEAKING O-RING SEAL WAS REPLACED BY A LICENSED TECHNICIAN AND THE BRAKE WAS GND TESTED OK. AFTER THE REPAIR; THE AIRPLANE WAS FLOWN FOR ABOUT 40 MINS AND THEN LANDED. THE AIRPLANE WAS ALLOWED TO ROLL OUT WITH NO BRAKING BUT APCHING THE SECOND TURNOFF THE BRAKES WERE APPLIED AND THE AIRPLANE SWERVED TO THE L AND DEPARTED THE RWY. NO DAMAGE WAS INCURRED. THESE BRAKES HAVE A MAINT HISTORY AND HAVE BEEN REPLACED 3 TIMES. INVESTIGATION OF THE FAILED R BRAKE REVEALED THE BRAKE CYLINDER PISTON WAS CRACKED ALLOWING BRAKE FLUID TO BYPASS THE PISTON. DURING THE PROCESS OF GETTING A NEW CYLINDER PISTON IT WAS DISCOVERED THE MANUFACTURER HAD MODIFIED THE BRAKE SYS AFTER THE EARLY SERIES OF ABOUT 35 OR 50 AIRPLANES. THE MODIFICATION CONSISTED OF HEAVIER BRAKE ROTORS; LARGER AND THICKER PADS AND LARGER BRAKE CYLINDERS. THERE WAS NO NOTIFICATION TO OWNERS OF THE EARLIER SERIES OF AIRPLANES OF THE BRAKE SYS MODIFICATION. THE AIRPLANE NOW HAS THE MODIFIED BRAKES.