Thanks for that link. Lots of good info there, I like the animation of a hydraulic jack.
He says that leaks at the main piston indicate problems further below which is what I was thinking might be the case. However the Happijac's may function differently. When I removed the main piston/ram I didn't see any obvious sealing devices at the bottom, there was a large plastic piece called a bushing but I didn't see any real sealing function.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13