I noticed my AC was not getting real cold. It was converted to R134a years ago before I bought it. I check the pressure and it was low. I added some freon and when I pulled the hose off the fitting it was spraying out the freon I just put in it. I got a new R12 to R134a fitting and pulled the one off that was on there. It did the same thing with the new adapter on it.

Does the R12 schroder valve do the sealing and the adapter just push it when you fill it?

Can the schroder valve be replaced without pulling the metal piece out of the AC system?

Do I need to pull a vacuum before filling it again since it completely vented?

Should I buy just the plain 134a freon or the one with a little oil in it?

What should I check next?

Thanks

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85 300D Turbo

The Schrader valve can be replaced without having to evacuate the system if you catch it soon enough. There is a tool for that available at AC supply houses...
You did not catch it soon enough so you need vacuum before reinstalling your refrigerant..
and since it is discharged by the leak you do not need that special tool... just put a new valve in and then vacuum.
Was the leak at a low spot which caused you to think you lost oil from the system ? If yes it probably would not hurt to put part of a can of the combo R134a /oil into the system before your main charge.
I do not know which metal piece you are referring to...

The piece where the original R12 low side fitting is on the system. It looks like the original fitting does not thread into the metal block that the AC hoses go through on the system.

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85 300D Turbo

Picture ?

Try using a screw driver and pushing the valve all the way in briefly and releasing it. Like just tap it so it opens for a fraction of a second. Doing this reseats the valve and can make it seal.

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1981 300SD 512k OM603

I had an identical problem on my 134A converted system. Let me see if I can explain what is going on with yours and if you have the same problem. When the adapter is installed you have (2) Schraeder Valves stacked. The bottom one is the original one that will stay open when the 134 adapter in put on top of it. What I found with mine was the top Schraeder valve seems to adjust itself with vibrations; vibrating CW over time. All I had to do was slightly adjust the Top schraeder valve so it would seat. You can do this with a Schraeder valve tool or just use a small screw driver that fits to either side of the valve down the hole. I believe you will need to adjust it up slightly CCW till it seats and stops leaking. For me this was less than a full turn. Now it is fine.

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1982 300SD

If you had pressure in the system, even if just a little bit, you may get away with not vacuuming the system. Your low pressure cut out swith should keep it from pulling a vacuum if you are sure it works you may be ok. Try to avioid putting in additional lube if you don't see allot of oil leakage at the point where it leaked. Once you get a can or two back in the system you can push open the Schraeder valve with a screwdriver and see how much oil is returning to the compressor. Mine sort of looks like shaving cream coming out of a can, maybe a lil bit less, but allot of oil. If you don't see a bunch of oil in the return to compressor you should add some. (2) small cans of regular regrigerant should get you to the point where you can observe the return of oil as mentioned and you can add oil from there from a pressurized can.

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1982 300SD

Does the '85 have a LPCO? My '96 does not and our old '83 did not.

Ensure that the adapter is not leaking at the threads where it connects to the old fitting. If not, if the top is smooth, put a cap on it.

Fittings always leak if you don't cap them. Be sure that there is an o-ring in the cap for best results.

I suspect that low pressure protection is lot more common than you believe. I have personally never encountered an a/c equipped vehicle without it. Most R-134a models also have high pressure protection, usually from the same switch.

The cutout for low pressure is on the high side. Thus, it will prevent the compressor from starting, but it won't shut it off once it gets going unless you manage to lose nearly all of the refrigerant.

When those conversion fittings are put on top of the old R12 fitting isn't some type of sealant supposed to be used ? Like Red Locktite except more permanent ?

I bought a new adapter and there was a thread sealer already on the new adapter.

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85 300D Turbo