Greetings, and Welcome to this site,

The part you are describing is the fuel primer pump. It can be ordered here at this site for less than $20 and will cure your problem. The o-ring in these older designed primer pumps just get worn out and cause them to leak. If you order the Bosch new design I feel that you will have a trouble free unit for quite a few years as it is redesigned and works much better. If you are unclear which part to order just post your year, model and make and I will get the p/n for you off this site. In the search you can use primer pump entry which should get you there.

Charles

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"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan