header tank leak

when you are dealing with fuel systems, there is no such thing as simpler or cheaper. If there is one thing that can kill you faster in a plane, I don't know what it is. PRC is the industry standard for fuel tank sealants and it does not take much to seal a header tank.
the fuel tanks in the Ercoupe were sloshed from the factory, it was the best they had then, but sometimes the sloshing compound starts to peel off from the tank walls, and if the piece is large enough, it can block the finger screen. PRC is the best thing there is.

Nathan, Or spray Carb cleaner on and around the fitting . that will remove all the crap off the tank wipe clean then do the testing. have old towal to catch the cleaner on the floor. and one to wipe the fitting and tank. Spray the shut off valve also.

Yes, if you are referring to Matts' info below. He's the "Guru" with all the experience and so helpful to all in sharing - follow his step-by-step instructions.
The old days, ProSeal was one product IIRC. They have more products under the name - so make sure the one you select is the right one. If any question, suggest you call them and discuss the application.

Ed, I'm definitely not an AP so I speak from hearsay only. The way it was explained to me, I remove the tank and drill out the rivets to separate the top half from the bottom. I clean up the inside, patching any holes, then I lay a bead of Pro Seal in the seam, kind of like Liquid Gasket, before I rivet the two halves of the tank back together.

Is that how you understand the Advisory Circular? Or am I misunderstanding the procedure?

Suggest you'll read AC43.13-1B,page 8-19 ",Fuel Tanks and cells" - "special sealing compounds are available and should be used in the repair of tanks". I think this permits one to go the simpler route vs the $, such as using Pro Seal, etc. 2c

Reporting back . . . 

I did a ton of research and so did my AP.  We came to the same conclusion as you fellows did: sloshing is not the answer.  Safe Skies in Ohio sent me a detailed quote for repair, around $2,000, and they have great references from other Coupers so it looks like I have a winter project!  

Many, many thanks to everyone on this list.  Truly amazing resource.