Next to where the pick-up tube attaches to the pump body you will see a fair sized silver bolt with a hole in the middle. That is the pump relief valve. Take the bolt out...a coiled spring is under it. Put no more than 5mm thickness worth of washers between the spring and the bolt. Obviously, the washers have to be the same ID as the spring housing. Make sure you use washers or a 5mm thick spacer with a hole in the middle the same size (or larger) than the hole in the bolt.