I would think that if the tank is the original and you are using an original pick-up, this shouldn't happen as it would have been a problem since the car was new. If you can, take the sending unit back out and plug the input end of the pick up tube . Submerge the unit in a bucket of water and blow air through the output end of the tube . If there is a leak, you should see bubbles in the water.
I'm thinking it may be leaking where the tube going into the tank is soldered to the piece that holds the sending unit to the tank.
