AC expansion valve replacement
 

My 1990 W124 2.5 diesel AC system leaks out in hours. The leak is not in the usual suspects which are easy to inspect in the engine compartment. My guess is the problem may be the expansion valve, which has 4 o-rings and is tough to get to. If this makes sense, how does one get to it? Go in from the top by removing it per the 83-140 air inlet removal writeup in the online manual? That would seem to be reasonably straightforward except for the possibility of breaking 30-year old plastic parts. Any experience/ thoughts you might share?

Don't guess. use a halogen leak detector or ultrasonic leak detector.

W124's have a history of leaking evaporator coils. Use a leak detector or a dye to look for the leak. If you're leaking out in hours, be looking for oil, it should lead you to the leak.