ok so i bought a used ac unit from a guy and im currently istalling it.... my questions are: how long should i run it on the vacume, and then should i get a new one of those evaporators (the beer can looking thing uder my powersteering).

 

I usually hook it up for at least a half hour, but it depends on how good the vacuum pump is. If you have a vacuum gauge, let it get as close to 30 inches of mercury as possible, and leave it there for a while. The crappier the pump, the longer I leave it there.

You shouldn't need to replace the evaporator if it's good. The beer can thing you're talking about is the receiver/drier. If the system has been open for more than like 6 hours, you technically replace it. If the system has been open for several days or weeks, then yeah, go ahead and replace it.

Also you want to be careful about the oil you use and the refrigerant. If it's from 93 or earlier, it will be R12 and as such, have different oil and connections than the 94+ R134A system. If the A/C you're using is from 94+, you're golden on the conversion. Otherwise, I'd suggest you follow the general instructions in something like http://bernardembden.com/xjs/134/index.htm or this http://www.nichols.nu/tip306.htm