I'm having a problem with a Hills Mk 3 hand pump which I'm using to pump up a HPA converted Ratty. For some reason when I've gone to refill the gun the upward stroke has become unbelievably stiff (to the point that it's impossible to pull it all the way to the top of the stroke) and then the downward stroke isn't 'biting' properly and it's not opening up the fill valve in the tank. Any idea what's going wrong here?