Have a 15 year oldish US Products portable. Has a heater and adjustable pump on it. Been sitting in the garage a while and decided to pull it out and clean it up. When I turned everything on the pump just kept building pressure , I would pull the trigger and it would come down to normal adjusted pressure. As soon as I would let off it would just start climbing, to the point of it blowing a pump pressure line. Any idea of what’s going on, when I last used it it was fine.