I would try again with another new sealing ring. Sop up the fuel in the bore and see if there is a hair on the surface - even that will cause a little leak. The surface on the bottom of the delivery valve too might be gouged allowing fluid to seep by through the groove. Clean it with rubbing alcohol and coat with some prussian blue. If it still leaks you can remove again and maybe the marks against the bluing will tell you something.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.