I am working on charging a pack howitzer's recoil oil. Nitrogen is charged and that went well. I fill up the oil reservoir, the oil indicator sticks out like it should, but when I take off the oil filling hose, recoil oil just pours back out. I thought there was a check valve on the inside of the gun that kept the oil from coming back out? is my check valve just froze? If so, how to unfreeze? Thanks for any guidance.