Sounds like you may have a crack/ hole in the line or the pickup tube in the tank may have broken off. Does this happen continuously or is it just below 1/4 tank or so? It could be an o-ring where the line clips onto the tank or on the back of the selector valve also.
Could also be the line from the selector valve to the tank, but it's probably in the tank. The line on my sending unit broke when I was replacing the tank. I was able to heat the old one where it goes into the tank to melt the solder, then solder in a piece of 3/8 steel line to replace it. Ran the steel line to the bottom of the tank to eliminate the shower head/ cone of failure.