w140 92 300sd need 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
No it should not be pressurized the tank.



