Air bubbles after the filter of TD5

my defender won`t start anymore. this is a TD5. i hooked up some transparent tubes before(low pressure feed to the filter head) the fuel filter and after it too. low pressure feed line to the filter is very clean without any air bubble. But the return line from the fuel filter(low pressure)which is going to the pump is full with air bubbles. Note : I have removed the return line from the engine and air tied the union which is relevant to that in fuel filter head. That return line hooked up to a basket.
How is the air bubbles getting in, after the fuel filter. checked the filter gasket, even fitted a new filter and also checked the drain plug in bottom of the filter. and also the air bleed valve is okay. don`t know how to test it, diesel is not coming out. so my guess is that`s a good one.
Pump is noisy. changing pitch according to the air bubbles which i see after the filter (output of the low pressure line from filter to the pump).
When i directly connect the hose coming from the pump (low pressure feed) to the pump again(i mean without going through out the filter), no sound at all + not a single air bubble. And engine runs without a hassle.


Thank you. any help would be appreciated.

Have you recently changed the fuel filter?

no actually.

knackered o ring or the fuel filter has a pin prick hole?

is the fuel filter still tight on the housing?

itll run like a dog if your getting air in there, check your unions are tight and maybe replace the crush washers too

fuel filter is new and it is tightened up too. this is a headache now. I can`t understand where the hell is getting these air bubbles.
Do you know any procedure to check the air bleed valve.

Is the sediment drain on the bottom of the filter good?
Filter only needs to be nipped up, to tight is as bad as to loose.
Filter heads can also go porous.

Make sure there is only one O ring on the top of the filter. Sometimes the old one sticks to the housing and it wont seal properly with two O rings.

filter housing s corrode and let air in change filter housing ,,,,,happy days