hello guys I'm new in w140 body and diesel also so i need your help

i got a good one for you

I'm a mechanic and i owe a shop but this one is new for me never have any thing like it

the car is start acting crazy its start every time runs like champ for few seconds then start slowly dies and i can do that all day long fills like not enough fuel so i change fuel pump(used one) and fuel filters 2 if them main and secondary
but when i was changing a main filter i deed find out that the car can run all day i was thinking that its fixed but i was wrong

so to bring the car back alive all i have to do is untaught a main filter then thought it buck up and it will run

if i let it sit over 10 min i have to do it again

any idea what is it ???

2 thinks (1)fuel tank have a air pressure is it normal ???
(2)mi vacuum pump is dead (under rear seat)is it going to effect that ???

so pleas let me know if you can help me

there's another filter in the tank that likes to plug with age.When changing filters make sure they are full with diesel or ATF.Thats goes for primary and secondary.Other wise air in the system will shut her off.Make sure you have good flow from tank first.No it should not be pressurized the tank.