Actually, he probably needs 134 gauges. Something about a sticker saying "converted to 134a."

So...read this page: http://www.aircondition.com/tech/questions/82/

Decide whether this is something you want to troubleshoot yourself, or pay someone to do for you. If you're going to jump in, then get some gauges, take some pics of your compressor, take some readings and report back. Harbor Freight advertises a 134 manifold set for $50.

You may run into a situation where your car was "converted" but they only swapped out the low-side valve to a 134 valve, so the gauges may not hook up. I have that and a few others report having the same thing. If so, you'll need to get an extra high side port to add a place to hook the 134 gauge. It's another few bucks at any chain parts store - maybe $10-15.

If, like Larry says, your car uses an older-style York compressor and yours is bad, you can find them here for $360 http://www.allpartsexpress.com/ShopByVehicle.epc?q=1979-MERCEDES--BENZ-300d--/--L5_3.0l_diesel-Climate--Control&yearid=1979&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1193699%40%40300D++%2F+L5%5F3%2E0L%5FDIESEL%40%40300D&catid=Climate+Control&subcatid=A/C Compressor&mode=PA

(That's a link to All Parts, a site sponsor, IAW the policy on linking parts to the sponsors).

That site also shows an R4 compressor, so maybe yours has the R4. There are cheaper sources for other compressors, but Phil gives good service, so give him a shot at supplying your parts.

You can get adequate cooling with 134 from a W123, but the experts will say the best cooling comes from R12.

Cooling effectiveness of the refrigerant is one factor, the effect on the life of your compressor of higher pressures required by 134a is another factor. You'll get opinions on restoring back to R12, or just staying with a 134a conversion. It's your car, in your neighborhood, with your money; so ask some questions and choose what works for you.

If your system is dead and you want to restore it. The Diesel Giant pictorial on flushing your system is a good reference. It uses Freeze-12 as a refrigerant, which will generate some controversy, but covers the basics of a thorough flush. It waaaaaay understates the mess associated with flushing, but it's nothing a competent DIYer can't handle.

Since you express a concern about eco-friendliness, don't vent your system, regardless of 134a or R12 (it's illegal, and messy when pressurized oil vapor sprays out) capture your oil - you may want to measure the quantity anyway. A shop can recover/vacuum test your system before you crack any lines open. You'll pay a little, but you're doing the right thing.

Note: spraying pressurized 134a-driven oil vapor refers to my experience with an accident damaged condenser, not illegal venting. My "what is this cr*p all over everything..." experience with removing oily front-end parts gave me an appreciation for what a little oil and pressure can do.

Wear goggles.