Chiming back in...
In my case, being a DIY A/C technician means not being the best citizen for the environment. Most of the refrigerant in the system had already leaked out through the rusty accumulator, I went ahead and vented the rest. R134a isn't as bad for the ozone as R12. While I wish I could have recovered it, it just wasn't feasbile.
If your system is almost dry, then releasing a few ounces of R134a probubly isn't that bad. Not great though. Releasing a full system to change a component, that's a tough call - I don't think I'd do it.
When you release the refrigerant, air will enter the system and the system will be at zero psi. Not pressurized, not a vacuum, but still "full" of air. The system is vacuumed so that it is truly
empty. Any air remaining will decrease system effectiveness. I was not able to get all of the air out - though I got most
I used a venturi pump on my air compressor. It worked - I can sit in 90 degree heat, in the sun, with a black car, in stop and go traffic, and be so cold I have to turn up the thermostat. However, I've found that all windows and the roof
must be closed and the control on "Max A/C" otherwise it just can't keep up. I couldn't find an electric vacuum pump to rent or buy cheap enough. But - if I had one to use, it would have been a no brainer.