WHEN OPERATING VEHICLE AT LOW SPEEDS THE BRAKE PEDAL BECOMES STIFF AND HARD TO PRESS DOWN. THIS CAUSES THE TRUCK TO CONTINUE MOVING AND EVEN CAUSED ME TO REAR END ANOTHER VEHICLE CAUSING REAR DAMAGE TO THAT VEHICLE AND A SMALL DENT IN MY FRONT BUMPER. THIS ISSUE DOES NOT HAPPEN AT HIGH SPEEDS BUT 15MPH AND UNDER IT OCCURS.