I have done a few R12-R134 conversions successfully and here is my experience. I do not normally flush the system as I am a shade tree A/C guy. The results may vary so it is your call. The steps I take:

1) Make sure the existing A/C works, I mean the compressor will come on when you call for A/C. You may have to jumper the low pressure switch at the receiver dryer to simulate a charged system, run it for < 30 secs.
2) I ALWAYS replace the rec/dryer when I do the conversion.
3) Pull vacuum and make sure it holds for a few hour, if not fix the leak.
4) Put 8oz of ester oil in system. ester oil will mix well with whatever was in there.
5) Replace or screw on new valve to convert to R134 fitting.
6) Charge with R134, A/C on and fan at full speed.
7) I do not measure pressure as such even though I have the manifold gauge.

Always wear safety glasses. Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.