I'd check to see if it's a line first. If so, it's a relatively easy fix. My '83 developed a front seal leak after about 300k miles. My solution was the manual swap I had been contemplating for the previous couple of years. It's considerably more involved than just pulling the trans, replacing a seal, and stabbing it back in, but in my opinion it's well worth it. A lot more people have done this swap in the W123 as opposed to the W126, but I think I remember seeing one or two 126 manual swaps discussed on here at one time or another.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.