THE AIRCRAFT WAS UNDER THE CONTROL OF A RECENTLY CERTIFICATED PRIVATE PILOT, TAXIING FOR DEPARTURE. HE SAID HE WAS TAXIING 6-8 MPH AND WHEN HE TRIED THE BRAKES THE LEFT ONE FAILED AND THE RIGHT WORKED, VEERING THE AIRCRAFT OFF THE TAXIWAY AND DOWN AN EMBANKMENT. THE FAA SUBMITTED A DRAWING SHOWING THAT THE AIRCRAFT LEFT 31 FEET OF SKID MARKS ON THE RUNWAY AND SLID AN ADDITIONAL 42 FEET ON A GRASSY AREA PRIOR TO GOING DOWN THE EMBANKMENT. WHEN THE AIRCRAFT WAS EXAMINED IT WAS NOTED THAT THE BRAKE PEDALS WOULD BLEED DOWN IF SLIGHT PRESSURE WAS HELD ON THEM. HOWEVER, THEY WOULD RECOVER WITH ONE PUMP OF THE BRAKES. THE PILOT HAD 2.5 HOURS IN MAKE AND MODEL WITH 1.1 HOURS AS PILOT IN COMMAND. THE FAA EXAMINER WHO FLEW WITH THIS PLT ON 2-5-88 FOR HIS PRIVATE PLT CHECKRIDE MADE A CRITICISM THAT THE PLT TAXIED TOO FAST.