I bought a used one from a shop as well. Got a pretty good deal. The manager didn't know what he was selling. For $100 I got a charging machine with a 30 lbs cylinder with about 5 pounds in it, a recovery machine with about 30 lbs in the recovery tank.

The recovery worked well for about the first three times I used it. I think it took in to much oil and stopped working so well. I think the filter plugged but I haven't needed to use it since so it is still sitting in my garage as is.

When it was working it only took a minute or two to empty the car's system. After what I assume was a plugged filter, I noticed results similar to yours.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD