You will have to pump while someone starts the engine. Once running, tighten the cap down. If the cap leaks, you will have to replace the hand pump. They tend to fail when not used.
We have them in the partsshop.
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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope

1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM