Amongst my collection of oil guns is Wanner 300-4, It forces oil through the nipples with some force, good. But I cannot stop the oil from leaking back from the filler cap end, the pressure is so high it forces oil back up the screw cap end threads. So far, I've made a new plastic washer, tried an O ring with a large diameter washer made to fit, all to no avail. I have had the gun in pieces, nothing wrong at the action end. What am I missing. John