header tank leak

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!

If you can pressurize the tank with a low pressure (I use a balloon as a pressure regulator), then use soapy water to identify exactly where the leak is at. Then, if you can get a small low pressure (vacuum) in the tank (not too much, don't collapse the tank), you could maybe paint a sealant over the leaky seam and suck it back into the leak to seal it. Just a thought.