Cattledog said:
Do you have a value? Deep is a really non specific value.
The pumps I have can run from 0.001 torr (oil rotary vane style) or 7 torr for Piston - positive displacement-. I think 0.001 torr is overkill. 10 to 50 torr should be fine for this application. Any thoughts?
BTW, 10 minutes even on the oil rotary vane style pump will not dry the system. To really dry the system you would need to measure a value of less than one torr on a guage. My guess would be about 24 hours with a dry ice cold trap. I would let the ac dryer worry about the water in the system.
The purpose for pulling a vacuum the first 10 minutes is to establish a "baseline" to watch (not to dry the system). In other words: I need some amount of vacuum on the system to check and see that it doesn't leak down significantly. If it does...I've still got a leak somewhere and continuing would be akin to "pissing into the wind".
What we're doing by applying a vacuum is: Removing any air in the system and lowering the pressure to a point where moisture will "boil off" and can be removed as a vapor. This is a very important step however.
Vacuum Level Required to Boil Water.
At sea level, water begins to boil and change into a vapor state at 212 degrees Fahrenheit. If we increase the pressure we can raise the boiling point of water.
If we wish to lower the boiling point of liquid, we simply remove the pressure that's on top of that liquid. That's how we boil water out of an air conditioning system. We use a vacuum pump to bring the system to a level of near perfect vacuum so the water will boil off and be carried away as a vapor.
It's important to note that ambient temperature has much to do with the point at which liquids will boil under vacuum. The greater the temperature, the fewer microns of vacuum will be required to start the boiling process.
Here's a chart that shows how temperature plays a role in the level of vacuum needed to boil water.
Inches of Mercury
Boiling Point of Water °F
26.45
120
27.32
110
27.99
100
28.50
90
28.89
80
29.18
70
29.40
60
29.66
50
29.71
40
29.76
30
29.82
20
29.86
10
All values are at sea level. Subtract 1 inch for each 1000 ft. above sea level
What I mean by "Deep Vacuum" in layman's terms is: (For most places) at least 28" of Mercury for at least 30 minutes (an hour is better).
If you don't start with fresh oil in your pump (if yours requires oil), then you're unlikely to get the best vacuum you can.
Without a micron gauge...you don't really know how much vacuum you've pulled, but for the average automotive application simple manifold gauges will suffice.
Whatever you do, if you have opened the system, or it has completely leaked down...replace the drier and pull a vacuum. Don't just "pour in the cold".
. too much 
