Diesel tank issue (i think ).

Managed to get my 1.6 diesel running after 10 years of being stood still .
I cant get it to run smoothley .
Im getting air  bubbles traveling back towards the tank in the line after i swithced the engine off  .
I removed the line from before the diesel pump but after the filter .
Fitted a inline filter and extended the fuel line so it could be dipped into a diesel can .
I started the engine and it ran fine and smooth,with no air being drawn after the engine was switched off .
So i replaced the fuel line going back to the tank with 8mm rubber reinforced fuel line .
Only to find im having the same issues again .
I think i have a issue with the tank as im sure if i removed the pipe from the tank and placed in a can of diesel it would be fine .

Ive checked the tank when i drained the tank and there is minimal rust in there .

Any ideas guys ?.