I have an Ex-Cell VR2500 pressure washer. After being in storage over the winter, I tried using it for the 1st time this season. The engine works fine and the pump builds pressure ok. The water supply was connected and water was flowing through the hose and wand as it was supposed to. When the wand trigger was pressed no water would come out. I had to slightly loosen the high pressure hose nut to relieve the pressure. This happened twice. The 3rd time I tried it, I had a friend hold the trigger while I started the engine. Everything worked fine as long as the trigger was held. As soon as the trigger was released, pulling it again resulted in no water coming out. The nozzle, hoses, and wand were cleaned. The water supply was checked. The unit was winterized as instructed. Is there a way for air to get into the pump and prevent the high pressure water from coming out once flow is stopped? Does the pump need to be replaced? It is the original pump, about 4 years old.