Bubbles in fuel pipe?

Anyone know why my fuel pipe on Vespa GT (1969) always has bubbles in it?
It\'s causing fuel starvation and won\'t go away. When I try letting the fuel flow before connecting to carb banjo there are always bubbles in it. It is fitted with standard pipe clips (new) and 24\" (600mm) of fuel pipe.
Tried tightening friction washer screws which bare down on the rubber washer untill I can barely opperate the tap. No luck.
Tried tightening the sediment bowl screw. No luck.
Tried fitting a new tap today… still no joy.
Have I done something wrong??
Can anyone help?

breather blocked or clogged or holed causing the bubbles.

Obviously drawing air somewhere along the line,have you tried another tank?

is the air hole on the petrol filler cap blocked

No, I get this even when the cap is open and when the tank is out of the frame!?!

No I haven’t tried another tank, but then I can’t see why it should make a difference.
The only time it stops is when I fill the tank above the top of the breather(?) pipe (the long one that sticks up) or if I block this with my finger???