I buy a brand "Master Chem". The stuff is made in Germany. I pay about $20 for an 8oz. bottle. You must be buying from a MB dealer.

Lots of great info in this thread.

I do question the comment above. If the system is under pressure, how can air from outside get in?

Its not air that get in - its moisture that get in and creates acids that eat out the aluminum from the inside out. If moisture could not get in then evaporator cores would not start leaking from internal corrosion. But it happens all the time.

If the system is under pressure, how can moisture from outside get in?

I wondered about that same thing for a lot of years. However, recently when I open two systems only to find the inside of the dryer to have streaks of rust on the inside of the steel container. My SL had a rusty residue on the inside of the sight glass. I thought "that cant be rust". But it was. And that dryer had more rust than I thought that it ever could have. I was shocked at how bad it was. Moisture has a way of getting into things that you would think impossible to do so. I have often wondered how it happens. Here is how it happens;

The short answer is; OSMOSIS.

Years ago GM used a plastic hose in their A/C systems that would "breath" using their own terms. It looked and felt like nylon. Moisture was able to penetrate the plastic and migrate directly into the high pressure area of the A/C system. There was a recall by GM to replace these hoses with newer material hoses. That was an extreme case showing moisture migration through hoses. GM has the labs to test materials and they verified that the moisture penetrated the hose itself.

Well today with the best hoses man can engineer, the rubber still is permeable by moisture. Its very very low compared to older materials but it still happens. Same for the O rings.

And so my friend the bottom line is; It gets in through the hoses, O rings and probably other places as well. Moisture always seems to get into places where you don't want it to.

I talked to a man in the business of nothing but automotive A/C repair here in the Houston. I respect his opinions. He recommends that the dryer be changed every 4-5 years. Sounds like good advise to me. I plan to follow his recommended schedule for dryer replacement. He also recommends adding 1 oz. of ester oil to the system because the ester oil "encapsulates" the moisture. I did a test myself where I added some water and ester oil in a jar and let it set for a week. You could not see any water after a week of sitting. The oil and the water were "one". I was impressed.

I use PAG 46 and it cost me for 8 oz about $12.00 US

Well everything is sealed, and my Vacuum Pump is running. I'll leave it on for 12 hours or so. Then see if it holds the vacuum for 1-2 hours.

I couldn't justify using such expensive PAG oil, so I used Castrol PAG 46. For about $10 US per 8oz. I was corrected by a MB tech, stating the system is desgined for 160ml OR 5.4oz of oil. Good thing, or else my cooling performance would have been reduced if I put in the 8.4oz as originally thought.

Well vacuum held at about 28". Don't have a Micron Guage.

I knew this would happen: How do I bypass the Pressure Switch on the Dryer, to activate the compressor so I can charge the system? Its a 3 pin switch variable resistor (which you can't jumper).

Anyone know how to active the compressor manually?

Disconnect the plug at the compressor and run a jumper wire to the battery and connect it to the clutch coil connection at the compressor. I use a clip lead long enough to get to the battery positive terminal. Be sure the connection is to the clutch coil. My compressors only have a single connection so you cant make a mistake. If yours has more than one, don't follow my advise. You might damage something. I know some compressors use a shaft speed sensor. Not sure about yours.

I have always just let the system recharge from the source of refrigerant without the compressor until it will not take any more gas. Then the compressor would always come on normally since there was about 90 PSI in the system. Never had to jump around any switches. But then again all my systems are R12 and older than yours.