WHILE DRIVING AT LESS THAN 25 MPH AND BRAKING THE TRUCK ACCELERATED UNCONTROLLABLY. ENGINE RPM ACCELERATED TO MAX AND TRUCK STRUCK A PARKED VEHICLE AT APPROXIMATELY 5-10 MPH. FULL BRAKE APPLICATION WAS UNABLE TO STOP THE VEHICLE. ENGINE CONTINUED TO REV AT MAX RPM AND WAS TURNED OFF WITH THE IGNITION SWITCH. THIS PREVIOUSLY HAD HAPPENED WHEN MERGING INTO HIGH SPEED TRAFFIC OR PASSING WITH ACCELERATOR PUSHED TO NEAR FULL DOWN POSITION. THIS TIME THE DRIVER HAD HIS FOOT ON THE BRAKE.