Honestly, if you are opening the system due to switch/oring leakage... you need replace as all the o-rings underhood at minimum ALONG with the receiver/dryer and said switches. Dryer must be replaced when the system is opened.

Now, if this is really your only problem (which it probably is not), you should be fine to recharge and motor on. W126s almost always leak at the expansion valve as most shops are too lazy to remove it to reseal it, then reinstall. This will REALLY test your patience in this heat, but the effort is well worth the reward.

Plus, how much oil is actually left in there due to the leakage? And WHAT oil is in there as you cannot mix oils? What refrigerant is in there after all these years?

It is probably not what you wanted to hear, but just doing it halfassed means you will be doing it again soon enough. I am doing my SD in this heat, only a few hours a day... but a flush, reseal, vac, and charge is really not that much work. I am being proactive by replacing my R4 before it dies as it is starting to make some funky sounds...

Lastly, please do not use any of the junk replacement refrigerants. Use R12 (do not worry, it is cheap all things considered), the M103 powered W126s are pretty much unacceptable in terms of vent temps in the summer with 134A or anything else. If you are totally anti R12, use 134A so at least you can service someplace in a pinch if you had to do so.

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