Auxiliary filter full of air, questions about the main filter

Hello everyone,

Today, I installed an additional filter in my Golf IV ALH.
'Because the diesel line is very tightly routed, I only saw the possibility of installing a Mann angled filter (WK31/11) horizontally behind the coolant expansion tank.'
to set.
In the process, I naturally had to shorten the rubber cable a bit.

Unfortunately, the air bubbles that are usually visible between the main filter and the ESP are likely coming from the tank itself. However, they are very small, about 1mm in size.

They are now accumulating in the pre-filter and cannot get out.

How did you install the additional filter? Did you run the diesel line? What about the air intake?

I've already checked the entire fuel line, loosened all the connections, inspected the gravity valve, etc.
I have no idea where that awful air is being sucked in from...

Subject: Main Filter...

Why is it letting air through?
Based on the sectional drawing, it can be seen that the suction line draws fuel from the bottom of the filter, which should be full of diesel and therefore acts as a reservoir.
Even if the thermostatic valve were leaking, the filter would still need to empty completely (there is a vacuum inside the filter) before the fuel level drops low enough for air to be drawn in.

Or could it be due to the RME? Does air escape from the material under vacuum, air that was trapped within it during the shaking process in the tank? When you shake oil, the air bubbles, due to the higher viscosity (compared to regular diesel, for example), also tend to dissipate relatively slowly.

This damn air is driving me crazy.

Who has an explanation?