I'm almost done repairing my A/C in my W123 and pull a vacuum on it last night for about 45 minutes before I called it quits for the night. The system seems to be holding the vacuum fine. I planned to finish the job this morning but some unexpected family plans have me tied up all day. I can finish the job tomorrow.

Is there any problem leaving system in a vacuum over 2 days? I know that the bearings in the R4 compressor are somehow sealed with the oil I put in the system so I wonder if having the system in a vacuum on so long might somehow eventually dry up those bearings and cause a leak. Thoughts?

No problem to my Knowlege concerning the Vacuum.

If you have some time inbetween you might look up if you need to change the Filter/Dryer in the AC System (if this system has one).

I have never done AC work on a Car but I have read that if the Dryer is exposed to Air moisture it starts to mess up the innards of the Dryer.

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84 300D, 82 Volvo 244Gl Diesel

I've been told that pulling a vacuum for several hours (pump on for that long) makes sure to get out any residual moisture in the system. I do that, but can't verify that it's imperative. Leaving a system with a vacuum seems better that releasing it if it's just going to sit for a few days. That would pull moisture back into the system.

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1985 300D-189k The 'UD', Ivory and Pinkamino
1979 300D-211k Dark Gray, Parchment
A 1980 Harley-~166k and
A 1994 Ford diesel pickup-349k and
A 1990 gasser Volvo wagon-145k

If you leave the vacuum pump running, then yes, you can evacuate for an extended period of time. If you are talking about pulling the vacuum, turning off the pump and sealing it off, that is NOT a good idea.

The reason is that if there is any leak no matter how small, you will be drawing in outside air, which always has at least a SMALL amount of mositure.

Since you've already done it and it held the vacuum well, then you have a LITTLE BIT of indication that there are no or minimal leaks. Evacuate again, before charging however.

If you don't have a micron gauge, it is very difficult to know if you have a leak with a manifold gauge. It's not NEARLY sensitive enough. Holding a vacuum even a few days with a manifold gauge will not tell you if you have a leak that would lose significant refrigerant in a month or two.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

You are correct in all this.

If you read most any auto a/c manual it will tell you to pull a vacuum better than 20 inches for 30 minutes before charging. I have always considered this to be enough to make the system WORK, but not arly negood enough to remove enough moisture to prevent corrosion.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

You are generally right in all this, but it sounds like you don't understand the reason behind it.

If a drier is exposed to air, the dessicate in the drier can easily get saturated with moisture. When this happens, there's no where for the moisture to go except to get combined with the refrigerant to form an acid. The acid then eats up the system from the inside out.

Whenever you replace a drier it is USUALLY done when the system is opened up to replace a component, fix a leak or something similar. When you do this ALWAYS get everything together and uncover and install your new drier the very last thing. Then IMMEDIATELY or as soon as possible, connect your vacuum puimp and evacuate. This way you get most of the moisture out of the system before the dessicate begins to soak it up. The less it soaks up, the more room in the dessicate to trap whatever residual moisture is left in the system.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

actually, it's best to flow nitrogen through the system before opening the dryer, so it actually has zero moisture to deal with, and the evacuation is just for removing noncondensibles.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

OK. I believe I have done the best I could within my limited scope of knowledge and things are OK so far. I had to skip the nitrogen pressure test because I just couldn't find an economical way to pull it off but the rest seems to have fallen in place:
0. replaced compressor, expansion valve, and ripped hose. Thoroughly flushed system and dried with compressed air.
1. added the new drier last thing and immediately pulled the vacuum.
2. left system in state of vacuum (pump off) several days)
3. ran pump again for 30 min or an hour
4. added freon (R12)
5. did my best to look for leaks with soapy water after the system was full of freon. I didn't find any. (If I had found a leak I'm not sure what I would have done since the R12 was already in the system. :-))

I was confused as I was adding freon because the high pressure gauge seemed stuck at 100psi as I was adding freon. Turned out that the high pressure valve seems to have malfunctioned? It was a new valve but I can't get either the low pressure or high pressure gauge to read anything from it all now. I guess that must have happened when the high pressure side reached around 100 psi. Not sure why that happened.

Also, it seems like the cans don't really empty unless I shake them or periodically turn them. I know I'm not supposed to do that but I did anyway since it seemed the only way to empty the cans.

The low pressure side seems to vary between 20 and 40. 40 when idling. 20 when revving.

vent temp was 42*F today on a 90+F Wichita day so I guess everything is working reasonably OK for now. I was expecting colder in the vent but maybe that's reasonable? The compressor seems reasonably quiet. On to the next set of problems I guess: (leaking tranny and oil, odo is intermittent.)

The whole site-glass bubble thing was pretty confusing too. I don't see any bubbles now when the system is operating so I guess it's either full or overfull.

It all ended up being more of a mystery than I had guessed it would be. The high pressure valve malfunction really threw me.

BTW I also had some problems getting the compressor to kick in without jumping it. That was an entire journey to itself, ending with me discovering that the OVP relay in my car is bad. I've jumped that for now and will order a replacement this week.

Thanks to everybody for the help and advice. This forum is amazing! :-)