Sealing access ports on stainless steel water & diesel tanks?

[mestrezat]

I need to open up my diesel tanks for inspection. No one has looked in them for 26 years. Want to get them all cleaned out and the fuel polished. Also need to open up the same ports on my water tanks. Have some type of air bleed into the water line screwing up the water pressure to my system. When the tanks are full, the tanks pressurize fine. When half full, the pump goes on forever. I need suggestions as to what material I can use to reseal the tanks. Some one suggested a neoprene material but I thought it might leave a strange taste in the water. Another suggested silicone caulk. Again, the chemicals.
Thoughts and suggestions please?

[David M]

For the fuel tanks, fuel resistant nitrile rubber and aviation grade Permatex. For the water tanks, whatever plastic access cap is already there and plain old silicon sealant. Correct, nothing harsh tasting for the water tank.