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On a Traveller - especially - a fuel-gauge sender leaking fuel is a PITA because it's inaccessible behind the wheel-well. Awkward items have to come off the car to access it. Starting with the 6 gallons of fuel. And then the tow-bar, the exhaust, and the tank itself.

Reading around, it seems that typically the leak will be through the retaining screw-threads, rather than the gasket itself. Especially if the "special copper washers" were omitted. Though it has to be questioned whether the washers really are that special because the possible torque on these screws - with only a cork gasket behind - doesn't seem nearly high enough to squash the copper to make a decent seal.

So I reckoned it was worth air-testing before and after the fix, to identify the leak and then to make sure the job's a good-un.

Most of the bubbles in the pic are just froth from brushing the soapy water on. The arrows show where it's actually blowing.
If you try this at home, I would suggest only sealing the tank neck using something fragile like cling-film (in the expectation that it would burst long before the pressure gets dangerous) and just puffing in tiny amounts of air to limit the pressure to a tiny fraction a psi. Otherwise it's just not safe. And anyway any significant pressure will cause the tank to change shape. (For example, even 1 psi would be far, far too much pressure. Just that modest pressure applied over the area of the bottom of the tank would mean 300 pounds of force trying to push the bottom and the top of the tank apart.)

Clean the threads on the screws and those in the tank, then re-assemble with sealant on the screw threads. This will not only help with sealing of these holes but should also stop the screws rusting in the tank.

Tried just cleaning the threads and globbing Hylomar Blue on them (after sticking a dollop of it in petrol for a couple of hours to check it really is fuel-proof). Was amazed when the air-leaks still continued around the screws even with sealant. Ended up building a sort of 'cone' of Hylomar under each screwhead, then screwing them in so that the sealant was squished by the screw-head into the hole beneath. Which seemed to work.

Think it's OK now. But I have ordered a set of the "special washers" (MiniSpares's name for them) and some extra-thick (3mm) cork/nitrile gasket material, in case it starts up again. Not that keen to use the washers... seems to me they're too hard to properly seal and they just add yet another interface between surfaces where a leak could happen. Wonder why they didn't use fibre washers?

An alternative idea I've seen is to thread the screws from the inside of the tank, do them up tight (guess by using a couple of locked nuts). Then use them as studs to fasten the sender with nuts. Seems likely to seal better. And has the possible advantage that you could just about get a spanner on the nuts to nip them tighter, even when the tank is fitted, if you needed to.

With the cork gaskets, you need to go easy on how tight you do the screws up. If they are over tightened, the gasket and the flange loose shape and they don't seal.

I make my own fuel tanks and make them to take a sender similar to these. I fit them with studs. I actually cut a 4 mm steel plate, face off the sender side, put counter sunk screws in from the back, which are then brazed so they seal. On fitting up the nuts (with star washers), I only tighten them about 1/4 turn at most.

Are the special washers annealed to help the seal?

Remember!

These tanks aren't pressurised, so when you blow them up to find leaks you will probably be finding leaks that aren't there at atmospheric pressure.

We tend to use Permatex Aviation Form-a-Gasket sealent on the gasket & threads, this being one of the only true petrol proof sealants on the market, off Ebay

In my working life we used to pressure test various assemblies, I agree that air pressure apart from health and safety issues can cause problems under test conditions, also air/gas is somewhat less viscous and far more searching than a liquid and could well give a false impression.
Sealant, gaskets fibre or rubber washers are all relevant to affecting a good seal but I would not necessarily rule out copper washers but first ensure that they are fully annealed and descaled before use.

I stop the lickage of my tank throug the screw threads by putting the screws inside out, that was my definitve solution, just like the pic on this thread.

http://mk1-forum.net/viewtopic.php?f=3& ... hilit=tank