IF YOUR TRUCK HAS REAR DISC BRAKES , THIS WONT APPLY, BUT IF YOU HAVE DRUM BRAKES , CHECK THE REAR BRAKE ADJUSTMENT , FIRST MAKE SURE THE HAND BRAKE CABLES ARE NOT TOO TIGHT, AND THEN ADJUST THE REAR BRAKES UP TIGHT ENOUGH TO COMPLETELY HOLD THE DRUM AND THEN BACK THE ADJUST MENT OFF UNTIL THE DRUM WILL JUST BARELY MAKE A SWISHING SOUND AS IT IS TURNED , USED TO SEE THIS PROBLEM ON THE OLD KS22 TRUCKS ON A GRAVEL STYLE WASHBOARD ROAD WHERE THE REAR AXLE WOULD GET TO JUMPING UP AND DOWN AND WITH THE REAR BRAKES A LITTLE TOO LOOSE AND THE LSPV VLV LINKAGE SET TOO TIGHT OR BINDING IT WOULD DO THE SAME THING , PEDAL WOULD GO DOWN AND THEN A COUPLE OF PUMPS AND GOOD HARD PEDAL, JERRY