CAV injection pump - vent?

Hab mal wieder was lustiges...

n ForkStacker has a 3cylinder Fiat diesel engine in it... Last autumn, Fathers threw this thing at me to lift something up on a higher floor... short time after start and bitsl the carrot simply goes out... Well... There are two diesel filters in series, both full with so much... Puddled all the stuff and new filters in... Hand pump (pre-conveyor pump) next tried, opened puddled... at some point I came up with the idea to empty the tank and put fresh diesel in it (in memory of an emergency power unit which was filled fresh, but obviously was ancient fuel) so hose in tank and sucks... I usually get nausea from the vapours, but not this time (??) and even when I had the face full of diesel (?????) well, it was also more water than diesel... So it was clear why he didn't want to run... And the plumbing was probably the diesel plague... jeah, so completely empty system, new diesel in - pound and then org... but??? NO MUCK!! [Censored]

Internet helps me in that the CAV injection pump still needs to be ventilated... Okay... But before there comes out something about the venting screws one can hear already in the return line in the tank splashing... so still removes the "intake" of the pump, there is a small filter and a valve in there... everything completely clean, there will have in the pump itself no smudge, immediately also looked whether the wing cell pump (pre-shipment pump) turns when starting - it does!... reassembly and want to vent again, still splashing into the tank... for test purposes, the return line on the second filter is unscrewed (last return line before it goes to the Pümp) and the thumb is held on it by the hand-pumping, now the airing is wonderful, it finally comes to diesel with the injection nozzles (a pipe only loosely screwed so you can see when there comes diesel)... SUPER!
Thumbs taken from the return port and tadaaaaa, a diesel geyser is formed and the pump is pressureless again...

What the hell is this shit???

then thought that there is probably a valve in the filter flange so that the return line opens only at a certain pressure... Nothing there, the return line is connected even in the pre-chamber of the filter... How can you get a touch of pressure? Why is that and must it?