Perhaps you recently replaced the pre-filter and didn't replace the o-ring? Pre-filters don't automatically come with new o-rings, you have to ask for and buy them separately. It's #29 in the diagram, goes in a groove on the pre-filter, then the pre-filter is pushed into its hole.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970