Now getting back to your original question about a homemade recovery system. It seems that if you had an old compressor that was originally in an R12 unit of some sort, you could simply use it as a vacuum pump to pump refrigerant into a recovery cannister.

This is really all that the recovery units do, except they have an automatic controller that measures pressures and actuate some valves to do it automatically.

The thing is though, you will have to thoroughly evacuate the cannister before recovery and know when the cannister is full. You do not want to try to add refrigerant to a cannister that is over 80% filled. Also, you need to understand the pressures and times involved for seeing that there is no non condensable gas (air) in the cannister.

I would not try doing all this unless you have a THOROUGH understanding of the pressures and so forth. Without THOROUGH knowledge you would be an accident looking for the right time to happen. This is why the machines or so common.

Good luck,