[Deadeye]

I've done a bunch of aluminum tanks in the shop, as we have to certify them before installing 'em. I've got a test rig with a ball valve and a pressure guage plumed into a tee. Then it's just a matter of bushing it to the fitting you're using. Cap the rest with pipe plugs and you're all set (assuming you can get a wrench to them of course!!). For us, they have to hold pressure for 24 hours and the soapy water trick is the best I've found for pinholes too. Don't worry about the pressure in your shop compressor - the volume of the tank is so high that it'll come up to a few psi (we use 5) slowly enough to watch and close the valve.

From what I've seen from refits, leaks are usually from fittings or rotten/chafed hose most of the time, not usually from welds. When the tanks is filled right to the gills, a bit leaks out till the fluid level is down the tank a little - that might explain why there's so little but, like you say, it doesn't take much.

Stainless water tanks will get pinholes but I personally haven't seen it with diesel (for whatever that's worth). I've never had to replace an aluminum tank for anything.