Help with fuel level sending unit replacement

Dear all!

I got my fuel level sending unit (for the posessed fuel level indicator) and installed it, as was wonderfully explained here on the forum. Everything worked fine for about a month, the sending unit went bad and I got a replacement from the seller. After I replaced the second one, i started to smell gas in the car and after a while I got "Gas lid not closed" message, along with the "Service Engine Soon" message. Basically to make the long story short, I realized that I did not tighten the ring that holds the gas pump tight enough. When I open the lid in the trunk, I can see that there's gas coming out of the tank, after the drive. I hammered the brackets that hold the metal ring as tightly as I could and the gas still leaks out when driving. The funny thing is that I did not tighten the brackets as tight the first time and everything was fine and now...
Please help - don't know what else to do.

The big "O-Ring" under the the top of the flange will need to be purchased at the dealer. Many of the aftermarket seals are too thin and will leak!

HotZ28, thank you for the response. The O-ring is original, I did not replace it. Can the ring lose its "holding power" over time?

Can the ring lose its "holding power" over time?Yes, if it is original, more than likely it has flatened out and has lost it's "holding power"!

HotZ28, thanks again for your help. Will hit the dealer tomorrow and will let you know the outcome.

HotZ28, hit the nail right on the head. I went to the dealer and got the o-ring for 18.21. Took out the old one, it had an inch long impression on the ring, that prevented ithe ring from being snug. Replaced the ring and vuala - the gas stopped leaking. Two days later, while driving both "Loose gas cap" and "Service Engine Soon" messages dissappeared, the gas tank got repressurized.

Thanks again for the help.