Your hoses are fine, no problems there (and R134 is bigger than R12, just lighter in molecular weight).

You do not have compressor engagement. If it's not running, even if you do manage to get the Freon in, it won't work. Check for a bad compressor clutch ground wire first, but it it is OK, check for contiuity through the clutch. If the coil is bad, no clutch. It can be replaced without discharging the AC, but I don't know if you really want to do it in the car or not, I've never tried.

You should get clutch engagement by jumpering battery voltage to the clutch wire -- watch to see if the front clutch plate starts to spin when you do.

I usually invert the first can of freon and allow the pressure in the can to drive the liquid freon into the AC, but I do this with the engine OFF. CCOT systems will tolerate liquid with no problem since the low pressue Schraeder will be on the accumlator, but MB uses an expansion valve system and they don't like liquid freon dumped into the compressor when it's running. One can will not leave liquid in the low pressure side, and I run it in on the high pressure side anyway.

I would stop where you are for the time being and fix the leak -- pull a good vacuum on the system, then shut off the line to the pump and watch the pressure -- you must hold at least 29" for at least 30 min, preferably indefinitely. I have the luxury of a very high capacity 100 mTorr pump, so I can get 30" of vac with even a moderate leak -- the normal small AC service pump will take half an hour or so to get to 29", mine will run down to 29 in about 30 sec. The key is having the system hold vac.

Check all the line connections -- a leak will leave residual refrigerant oil if it is very large -- this is how I knew the expansion vavle was leaking on the 300TE. I suspect the manifold gasket on the 300D -- this will be a big PITA since I cannot reach the screws on the inside with the compressor installed.

The normal conversion is to repair leaks, evacuate, and then add PAG oil and R134a. The mineral oil stays put (mostly in the compressor) while the PAG circulates with the Freon, providing properly lubrication. Ester oil will dissovle in the mineral oil, and if you do not have enough, will cause lubrication failure.

If you need to remove the compressor, drain the oil and flush the whole shebang, and replace the oil with ester oil, then charge with R134a. Charge ONLY to 85% of the R12 by weight, or it won't cool properly at low speeds.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!