EMPLOYEE WAS BACKING THE WATER TRUCK DOWN A PIT RAMP IN LOW RANGE WITH THE ENGINE RETARDER ON AND APPLIED THE BRAKES. THE PEDAL FELT SOLID THE FIRST TIME BUT THE SECOND TIME HE APPLIED THE BRAKES AN AIR LINE POPPED OFF AND HE LOST HIS BRAKES AND AIR. TRAVELING AT A HIGHER RATE OF SPEED IN REVERSE, THE BACK DRIVER'S SIDE DUALS CLIMBED UP THE BANK AND FLIPPED OVER.