I have two 1987 W126's, a 560SEL and a 560SEC, both of which had AC problems. Once the refrigerant was out of the SEC's system (it's fault, not mine), I disassembled every AC connection and put in new o-rings. That made a big difference!! The SEC had two o-rings in the compressor split, one pinched in a high pressure line, one fitting with two (2) o-rings in it, and several more just plain hard o-rings. Now I understand why it couldn't hold pressure!!

Went to AutoZone for their box of miscellaneous AC o-rings (green, viton), which was about $25. I use silicone grease as a neutral lubricant on all o-rings so they don't get damaged during reassembly. The SEC held 23" hg for an hour and I put in a little isobutane and the rest was propane. Freeze your buns off here in Vegas.

The SEL AC didn't work when I bought it in '03, but four new o-rings in the top of the compressor and two more in the freon line connector fixed it - maybe $5.00 worth of parts. I didn't know then I could use propane so I had it converted R134. It's six years later and every year or two I add a little R134 to the system to get it cold again at the start of summer. This winter that car is going to be returned to an R12 set of fill connections and then the isobutane/propane mixture. It's time to replace every o-ring I can get to as well.

__________________
There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered