On the pump there is a valve called a unloader valve, this valve is designed to open and bypass the water when you release the trigger so you don't damage the pump. Sounds like the valve is either stuck or defective. Remove this valve and clean the valve and the port that it sits in. This should clear up the problem. The valve is usually located next to the hose port for the gun. If you can't find it go to jackssmallengine repair.com and look up a parts breakdown. You should use a light coating of lithium grease on valve and port. Also, check the nozzle and hose for ubstructions and make sure the strainer in inlet for supply hose is not clogged and you have proper water flow with no leaks. Hope this helps.