Replace it with this, the updated one:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1MB12FET61QY16XCHK&year=1982&make=MB&model=300-SD-002&category=All&part=Primer+Pump

If its leaking fuel it can or will leak air. Also a small air bubble in the prefilter is normal, and with use will shrink a little.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200