Whats a Primer Pump
 

??

Guessing that you might have a diesel (you didn't include a model) the primer pump is used with diesels to get fuel to the injector and to purge air in the system.

Haasman

In what context? In MB Diesels through 85, it's a device attached to the primary pump on the injection pump. Its purpose is to fill the system with fuel when air seeps in. You typically use the primary pump after replacing the main (spin-on) filter.

Sixto
95 S420
87 300SDL

Oh is that what it is, so only diesels have a primer pump? I have a 300SD btw.

I don't know if mechanical injection pumps for MB gassers have primer pumps. The last US-spec production MBs with a mechanical injection pump would be the 72 280SE/L and 300SEL 6.3... I think.

Sixto
95 S420
87 300SDL