AC evacuation question
 

I want to evacuate my AC system to check for leaks before I put that expensive R-12 back in it. I have asked around of my friends and no one has a vacuum pump. I thought I'd buy one but there are so many variables that I couldn't decide. I see one that pulls 2.5 CFM and a better one that pulls 4.5 CFM. Anybody shed some light on this subject?

Check at Autozone. When I did my AC, I used one of theirs that was in their tool loaner program.

I also got the HF one... the only suggestion is to be sure to get a TWO STAGE pump...
while the number of cubic feet per minute will make a difference in the time needed to do a good job... it can be left a little longer..
BUT if you do not have the suction power of the two stage pump you will simply be caught short of ability....

x2

Try to find an old fridge or small A/C wall unit. If you have a bulk garbage pick up day where you live you can usually find one at the curb. When you find one, cut one of the lines to the compressor. If freon leaks out, pass it up...it's there cause the comp died. If you find one with no freon in the system you've hit the mother load. It's there cause the freon leaked out, but the comp's probably still good. These compressors make excellent vacuum pumps. Solder a fitting on the suction side of the comp to connect your gauges to and voila...you've got a vacuum pump. Mine's been going strong for over 10 years now!
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firstly, checking for leaks with vacuum is foolhardy.
you need to check for leaks with pressure. nitrogen is a good way to test. pump up to 120psi and wait. check next day, if still at 120, you should be ok to evacuate and recharge. if it's low, pump it back up and start checking all the connections with bubbles.
vacuum testing for leaks is only possible if you have a digital micron gauge.
think about it. a/c systems are pressurized. so any leaks in a vacuum would be leaks that are not present under pressure. also, vacuum is at most a 14.7psi difference between atmospheric pressure... 120psi is MUCH more likely to show a leak.
thirdly, if you are leaking, vacuum will pull in MOISTURE FROM THE AIR!!! this will destroy the Dryer in the lines. and residual moisture will turn R12 into an acid that will cause more leaks.
seriously, test for leaks under pressure.
if you ever travel up to the charlotte area, I'd be happy to pressurize and evacuate your system. I've got R12 too... so I can even recharge for you.
I travel to Ft Jackson a few times a year, so I might even bring some things to you sometime...

vstech has it all down pat... all the correct physics and philosophy are there.

except for one concept. There is no way to check for leaks in the evaporator ( which is where one was in my 81 wagon ) with soap and bubbles..

Therefore..... you should use the nitrogen and FOUR OUNCES OF R22....
because the EPA knows it is better to VENT a little R22 as a matter of leak chasing and fixing than have people install R12 and then lose it through a leak.
The EPA approves the use of FOUR OUNCES OF R22 for pressure leak checking and venting it to the atmosphere ...

So the smart way to do this is pressurize with nitrogen and R22, CHECK FOR LEAKS WITH AN ELECTRONIC LEAK DETECTOR... ( it went off like a metal detector when they put it in front of the center vent... that was ten years ago... I know own my own ).
and then fix the leaks properly and proceed with the rest of the job.

That is a good trick and works very good. One of my uncles taught me that years ago. We both still use one.

An old refrigerator compressor does indeed make a dandy vacuum pump.

If you're not interested in rigging one from an old compressor, don't worry so much about volume, just make sure you get a wet vane pump. Lower volume will not hurt because a car system is very small in volume as refrigeration systems go.

An old refrigerator compressor is a closed system...
How are you people exiting the water vapor you are pulling out of the AC system ?
These vacuums you can buy at HF for instance..... have a place that the water will drip out of... and you can change out the oil .... which is recommended before each use... why would it be important to change out the oil on AC vacuums.... but OK to use old refrigerator compressors forever... which , being a closed and sealed system design.. do not have a way to exit the moisture ?

You simply leave the pressure port exposed to the atmosphere.

However, I would be worried that it would not pull an especially-deep vacuum. People using a discarded refrigeration compressor are also unlikely to own a vacuum gauge.

My 2.5 CFM 2-stage Mastercool pump is rated at 25 micron, but I can only get down to about 150 with the gauge dead-headed on the pump.

WOW, That seems like a pretty pessimistic view of the situation... I hope it is not correct....but it certainly could be given the parameters of the kind of thinking which would cause people to try to use an old refrigerator pump....

Doing a bad job of evacuation can cost a lot more in terms of future labor, new refrigerant, new receiver/dryer and potential leaks in really expensive items to replace like the evaporator due to formation of acid inside the system... than the cost of a new HF two stage vacuum pump and a set of AC gauges....

By vacuum gauge, I mean a thermistor gauge or similar. Mine's digital, but analog models used to be available.

Once I get below an inch (25,000 microns!), you can no longer see any movement on the manifold gauge needle as you approach 0. Never will you get to 0, of course.

If people will get the type vacuum which can be left on overnight... and leave it on that long...instead of trying to rush the process... that is a pretty safe method without having that type gauge....

not if there is a leak.
or if significant moisture is present.
or if the vacuum pump is not working properly.
nothing beats a micron vacuum gauge.
best practice is to evacuate to 2000microns, break with nitrogen, and re'evacuate to sub 1000 microns then charge with correct volume of refrigerant.
sub 500 microns is even better, but not needed, unless working with POE refrigerant oil.