[Vwdiesel] 92 Eco Diesel is running... Bubbles or holes

[Sandy Cameron]

If the filter is one of those with the white plastic thermo valve on top
that the return line passes through, thats the leak.

Almost every one I have owned has leaked  there at some time.
If it has been in use for some time, and then the return line position is
disturbed, the "O" ring, a part of the filter, not the valve, having taken a
"set" with age, will not seal properly, and will let air in.

It might be a bit of work, but you could splice a piece of clear line in the
supply line before the filter and see if the bubbles originate further back.