Is It The Compressor?
 

Year: 1981
Make: Mercedes
Engine Size: 3.0L
Refrigerant Type: R12
Ambient Temp: 79degF
Pressure Low: 60
Pressure High: 125

I just replaced my R4 compressor with a new (not a rebuilt) Compressorworks compressor, along with the receiver-dryer. I have thoroughly flushed the entire system twice. I also carefully added oil throughout the entire system, including 90 Ml in the compressor. I turned the compressor over by hand for 40 revolutions. The system held a good vacuum overnight. Total evacuation time was 4 hours.

When I started adding R12, I noticed that the low side was not going down as I would have expected and the high side did go up as high as expected. There was some cooling, about 5 degrees differential from ambient. One other thing I noticed was that both needles on my gauges were fluttering, something I haven't noticed before.

Which brings me to the title of this post: Is it a faulty compressor? Did I do anything wrong in the installation? I have installed about 8-10 R4s and this is the first one that acted this way.

Thanks for your help.

yes.i'm sure you didn't have a loose belt which is the only other poss explanation.so those pressures show a very weak pump[perhaps the reed valves].

I replaced the suspect compressor tonight with a known-good used compressor. That was the problem. As soon as the compressor clutch kicked in, the low pressure went down and the high pressure went up to the expected values with steady gauge readings. I figure that the suspect compressor had one or more bad reed valves.