During annual.

[JAG]

bking,
Those valves are easy to replace - just pop the wing tip off and it is right there. I have never taken it apart before on the C337, but if it is like other ones I have, it is a basically a small flapper valve in side with a light spring. Not very prone to failure except the spring may break. I might suggest taking the wing tip off and verifying that the arrow on the check valve is pointing the right direction (toward the tanks). If it is installed correctly, remove it and check it by blowing into it. It should free flow the one way and restrict the other. Still problems, next thing I would try is soaking it in in Varsol/Mineral Spirits) to clean it. Try cleaning and flushing it before opening it up. If you still can't get it to work properly, then open it up.. Basically one end screws off, and it seals with an aluminum washer. You can reuse the washer (may need a light dressing to smooth it out before reinstallation). Once you have the end off, you will clearly see the flapper and spring I am talking about. Hopefully, it is just some garbage (bugs or something) under the flapper. Just be careful not to damage the flapper and spring, it is really light duty to function properly. Let us know what you find!
Jeff