Mody,

I agree with the previous posters.

Consider that, if the tank has seen enough fuel go through, it may be time for replacing every tube, and maybe the stopper.

One way to verify cuts or holes in pick up lines is to connect a pump to it and to connect the jug to the vent of the tank.

Starting with a low level of fuel in the tank, just enough to cover the clunk, pump fuel out of the tank and observe if bubbles are going into the pump.
The vacuum that the pump creates inside the tank should keep the fuel flowing from the jug into the tank, which will keep the low level more or less constant.

No bubbles = Good pick up line
Bubbles = Bad tube

Just an idea.