It's on the transmission, attached to the right rear side of the transmission body. He always called it the secondary pump. He also thought there might be some leaking on the other side, around kickdown switch, but the majority of it was on the right side.

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1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.