Ideally the fuel circuit should be entirely free of air while the fuel travels from the tank, through the primary fuel filter, lift pump, secondary fuel filter, IP, and finally back to the tank.
Inspecting for air:
The clear line to inspect is the one with the T fitting on top of the fuel filter head.
The line is probably a deep brown color from fuel varnish if it is original.
If there is any air in it with the engine off, you have an air incursion.
If you see a stream of small bubbles in the line when you rev the engine, you have also an air incursion under load.
A small flashlight placed directly against the line can help show if there is air in an older fuel stained return fuel line.
Any opening of the fuel circuit, (like changing the primary clear fuel filter, activating the primer pump, or changing a secondary fuel filter) can cause an air incursion if not resealed properly.
Use new crush washers, o-rings, and rubber/Viton fuel hoses to properly reseal the fuel unions.
The usual suspects:
When replacing the primary fuel filter, the short rubber hoses should be replaced.
Stock fuel hose is metric (7.5 X 13.5) which is not readily available at the local auto parts store.
Many substitute 5/16 fuel hose which is a little loose and this can cause an air incursion, especially if
you don't use a primary fuel filter with bronze inserts (Hebmuller), and quality ABA or Norma style fuel clamps that are cinched down tightly.
When cinched tight enough, you should not be able to rotate the filter/hose union by hand.
I like to have the option of being able to run biodiesel so I've installed 5/16 Parker Super-Flex FL 397 hose on 'Desert Rose'
Unfortunately I have not found a metric equivalent...yet.
The '01' secondary fuel head hollow bolt needs a new crush washer and a new o-ring when you install a new secondary metal fuel filter.
The '02' secondary fuel filter head has a better sealing hollow bolt that uses two o-rings.
I have the ''02 ' fuel filter heads on my W116 300SDs.
If the primer pump leaks fuel when you manually activate it , you will get an air incursion as well.
The old style primer pumps are notorious for causing air leaks.
The black Bosch pushbutton style primer pumps are less likely to leak.
(Beware of inexpensive counterfeit Bosch primer pumps...they will leak)
Other less common suspects:
Pinhole leaks in the exposed metal fuel lines, especially under the rubber buffers in the brackets securing the lines to the body.
The clear lines attached to the IP can develop cracks, especially at the end of the barbed fittings.