header tank leak

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. 

if you remove the header tank, and it is riveted, resealing it is not hard to do, drill the rivets on the AFT bulkhead, work a flat utility knife blade between the bulkhead flange and outer skin, once all the sealant is cut, work the bulkhead off, clean all the sealant for ALL the seams inside the tank, you can seal the forward bulkhead and top center seam from the inside. Drill off the fitting on the bottom of the tank, clean off all the old sealant and reinstall with new sealant. you can stand the tank on the forward bulkhead and fill it with water and look for leaks, if there are none, drain the tank and let it dry out. lay a heavy bead of sealant just below the rivet line for the rear bulkhead and a matching bead on radius and just onto the flange, carefully push the rear bulkhead in place, install new rivets WET with sealant. Let everything cure and then leak check. any A&P can do the work.

Combine Raty and Super User suggestions. Proseal is excellent and I've seen tte vaccum used on Mooney built-in tanks in the wing, years ago. Certainly worth a try!