[wslade2]

We have had rain after rain here. The water collection occurred while tied down at shop for service by A and P (they're tight on space and move planes in and out-I go and physically check every couple days when outside).
My 337 is a 1970 E model. I have the standard cessna style cap that uses a 1/8 to 1/4 turn to lock. I found a drawing of the drain for the inboard tanks. It is called a scupper drain. For the inboard tanks, a metal tube heads aft and outboard until hitting a bulkhead/rib. It joins an AN fitting. Continuing on the other side after the AN fitting is a rubber tube. The rubber tube then joins another AN fitting that is in the "roof" of the compartment where header tank is at base of boom joining wing. Removed the access panel for the header tank and there is a short s-curved extension to the outside/bottom of access panel/boom. I found it easier to to remove this s-tube in order to clean the drain. Pipe cleaner good idea. I got larger longer fluffier ones from Hobby Lobby. At first the pipe cleaner would not go through. Some layered dirt had hardened in AN fitting which is an area of narrowing. Used some stiff steel wire with a 30 degree 1/16 length bend/hook on the end of it to catch/pull/dig on dirt. Took many manipulations to break it up. Put a small amount of water in scupper so I could tell when there was flow. Once dripping, ran the pipe cleaner through several times and flushed with water carefully. The outboard tanks scupper drain is a straighter shot and less trouble to clean. no s-curves. All were blocked. There are two drain tubes under each wing for scupper drains: 1 inboard and one outboard tank. Don't confuse with drain for boost pump which comes out through rectangular access panel near leading edge of wing (which makes for a 3rd tube poking out the wing bottom).
I replaced the gaskets on the caps. I could tell this helped alot as when one of the rains passed through, water was pooling in the scupper drain and not getting past gasket into tank. The old rubber gaskets "looked" ok but when manipulated showed cracking and brittleness. I would have a low threshold for replacing these. Cheap insurance.
Thanks!