Air bubbles in the fuel line?

Greetings everyone! I always get great help here on Fred's forum, with little problems the come up now and again. Thanks to you all! I'm going to run another one by you now. I have a 97 Passat wagon and have noticed lately that when I'm down to last 1/4 tank of diesel, I start getting quite a lot of air in the clear line from the fuel filter to the pump! It's a little hard starting at this point as well. I started to replace all the rubber lines, but got the wrong size hose clamps and had to stop. My wife needed the car and filled it before I could finish the job. When I got all the right clamps I took another look to see what was happening as the car started instantly as usual with a full tank, and not a bubble to be found! Does anybody have a clue as to what's going on here?

Some air bubbles are normal. But if your having a starting problem you may have a leak thats allowing air in and/or allowing fuel to be siphoned back into the tank. I assume you've changed your fuel filter by now.