I just got a 1992 300 TE with 134K miles, which had previously been converted to R 134a by an MB dealer. The system does not hold gas, obviously it has a leak somewhere. I put about a 1 lb in and 3 weeks later there is not enough pressure for compressor to kick on.

I've tried to find leak with my sniffer with no success.

My plan and I think this would apply to you too would be to rechange and add dye and then try to see if I can find the source of the leak.

I have another 1992 300 E and I replaced the compressor, expansion valve, reciever dryer last year. The compressor on that car was worn out and leaking. I hope that's the case with the 300 TE since I don't want to have to learn how to put in a new evaporator. The 300 E had a new evaporator installed before we bought it about 8 years ago when I had only 60K miles on it. That car now has about 126K miles pm it.

I think it best to add gas with engine running. If there is not enough pressure to do this you can add a little with the engine off and then turn the compressor by hand a few times to move the gas around a little. Do try to fill with the engine running.