You can do this several ways, one is to disconnect the drain hose (the one with clamps) and put it into a bucket. Disconnect it at the pump and put a cap on the pump tube or you'll have the contents of the resivoir on the floor. Then spin the pump freehand or use a drill with a bolt in the chuck and the fan belt to run the pump only, I used the pump belt before I hooked it back up. You will need to stop now and then to refill the resivoir. You can do this while turning the motor over and just as it starts shut it off (this is tricky and sometimes messy). It doesnt take long to drain the resivoir. Another way is with a vacuum pump hooked up to the drain side. Pump it until it flows clean. Either way, some of the crud that is built up in the box is stuck there and won't come out.
