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Pulling A/C vacuum before recharge- is this good enough?

Old 07-20-2007, 10:49 PM
Rob Edwards
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Default Pulling A/C vacuum before recharge- is this good enough?

I re-oringed my entire A/C system last night (front and back), replacing both expansion valves and the drier (whoever designed the captive nut placement on the drier bracket oughta be shot....) and re-assembled everything, (though I haven't rebuilt the two rubber-section compressor hoses yet)

I then pulled a vacuum for 1 hour to an indicated 24 mm Hg. (I guess my vacuum pump isn't as good as I thought...) I then closed both valves on the manifold gauge and let it sit overnight. 18 hours later, it reads 23 mm. Is that a good enough seal or do I look for leaks.

Also, it will be a few days at least until I charge the system. Is it okay to just leave a vacuum on the system for a few days, or do I risk trashing the drier?
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Old 07-20-2007, 11:44 PM
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I pull mine down to 28-30 hg and let sit. For your measurements it sounds like system is sealed.
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Old 07-21-2007, 02:21 AM
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Replace the hoses...

More vacuum is obviously better, longer time with the pump running is obviously better. The system would love to sit with the pump on it at least overnight, if you have that luxury.

Any moisture in the system that's liquid will s-l-o-w-l-y boil off to vapor, "reducing the vacuum" as you see the partial pressure of the vapor. This happens when there's a slight temp change, and will also happen as a little moisture starts to boil off and you have that localized cooling effect as the heat s drawn out of the surrounding metal. Eventually you'll find equilibrium, but it may take more than the hour you gave it.

I have a spiffy multi-stage lab-quality pump that sucks the needle against the 30" pin on the gauges. I don't have a torr meter to see how many molecules are left in there, but you get the idea. So you have 20% of the air still in there... D'ya think that will cost much? Wannna bring the car, your refrigerant, and a charging scale if you have one? A small group of us is going to meet tomorrow (Saturday), grill something, then go walk the Glendale Cruise Night show as the temps start to fall off. We'll park your car inside with the pump on it while all that's going on, then you can either charge it here or take it home, still under vacuum, and charge it there.

Absent that, leave the pump on it as you are doing other things. It's better to suck as much moisture out as possible to keep the drier dessicant from soaking it up anyway.

Call AM in the morning and get those hoses rebuilt!
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Old 07-21-2007, 11:32 AM
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The manifold pressure gage set is inadequate to measure the vacuum levels needed to evacuate the system before the fill. Moisture hides out in the lubricating oil and drier.

It's likely that your not pulling enough vacuum to remove moisture. It takes at least 900 microns of mercury (0.017 psi) in vacuum to remove water moisture. You could have small leaks that your gage set will not detect.

Professionals will use a thermistor vacuum gage to ensure the vacuum pump pulls enough vacuum to remove all water and flushing solvents, and as a check to ensure the system holds vacuum.

I have one of these:

but this will also work well:
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Old 07-21-2007, 02:27 PM
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On your 90' with rear air, there are a total of five hoses that can be rebuilt. Two at the rear a/c are easy to get to, but the small hose in the engine bay, leading from the condensor that must be done in-place without removing the engine.

On my 90' S4 with rear a/c, Partially prepped for new hose crimp....

New hose crimped one end....
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