Some of the locking rings that come with the new senders are not as thick as originals. If you still have the original locking ring,
It may be all you need.
I agree, I just did the same thing a few weeks ago and had the same problem. The new lock ring was not as good as the old one. I made a thin fiber gasket to go under the ring on the sender side and reassembled, problem solved. Also make sure all three tabs are seated properly, mine kept wanting to walk to one side allowing for uneven pressure on the seal.