Here is a good read on your check valve. Hope it solves your problem!
How to Remove and Clean a Check Valve
The check valve can be a huge problem. It is very difficult to reach and nearly impossible to remove if you don't have the right tools. It is, by far, the single most difficult part to work with in a lamp, lantern or stove.
The check valve is a small brass housing that has a check ball near the bottom, about the size of a BB. The check ball moves up and down inside the housing which equates to the check valve being open or closed.
When the check valve is open, the check ball is down and air can flow into the fount from the pump. When the check valve is closed, the check ball is up and it
blocks air from leaking back out of the fount.
The common fault of check valves is for them to leak. Any foreign substance on the check ball or its seat will cause a leak, because it cannot seat properly. This can happen because of dirt, oil from the pump, or from fuel reaching it from inside the fount. It is
not safe for pressure to leak from an appliance while it is burning.
With few exceptions, all Coleman® check valves are supplemented with an "air stem" which provides a second layer of protection against pressure leaks. It threads into the check valve and blocks air flow when turned fully clockwise. The air stem is turned by rotating the pump handle.
Most lanterns and stoves will have one of the check valve / air stem combinations shown above.