A C421 PLT; ON ROLLOUT AT HEF; EXPERIENCED A R BRAKE FAILURE RESULTING IN A RWY EXCURSION.

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CONTACTED MANASSAS TWR; CLRED TO LAND RWY 34R. NORMAL LNDG; LET ACFT SLOW DOWN WITHOUT USING BRAKES AT FIRST. LAST 1/3 OF RWY; BEGAN APPLYING BRAKES; R BRAKE PEDAL WENT ALL THE WAY TO THE FLOOR. L BRAKE NORMAL. COULD NOT AVOID BRAKING BECAUSE END OF RWY WAS AHEAD. WITH L BRAKE; ACFT SLEWED TO L; WENT THROUGH CUT GRASS AND ONTO TXWY V EXITING L FROM RWY 34R END. UPON ENTERING TXWY; ABLE TO CTL ACFT ENOUGH WITH VERY SLOW TAXI SO THAT TWR/GND CTL DIRECTED ME TO RAMP/FBO. THERE WAS NO OBSERVABLE DAMAGE TO LIGHTS OR EQUIP ON RWY OR TXWY AREA. FORTUNATELY; WAS ABLE TO AVOID ANY CONTACT WITH ARPT LIGHTS/EQUIP. CAREFUL EXAMINATION OF ACFT SHOWED NO SIGN WHATSOEVER OF CONTACT OR DAMAGE WITH EITHER GND OR ANY OBJECT. THE GRASS AREA WAS HARD AND GRASS WAS SHORT; SURFACE WAS FLAT; NO RUTS OR DITCHES. THE R BRAKE SYS WAS EXAMINED; AND SVCED BY ARPT MAINT PERSONNEL. THEREAFTER; BRAKING WAS NORMAL. THE ACFT WAS FLOWN AGAIN THAT DAY WITH BRAKES OPERATING NORMALLY. ALTHOUGH NO DAMAGE OR INJURY OF ANY KIND OCCURRED; FROM NOW ON I WILL BE FAR MORE ALERT TO THE POSSIBILITY OF BRAKE FAILURE AND MORE CONSERVATIVE IN APCH/LNDG IN ALL ACFT; KEEPING IN MIND THAT BRAKES CAN AND DO FAIL.