I thought about adding in my original post "dont even consider the HF vac pump".It will not achieve a low enough level of vacume,waste of money.If you try to purge rather than evacuate you will end up with intermitant cooling problems.do it right or wait till you can,otherwise you will do it several times and end up paying someone else to do it or living with an ac that dont work.I would suggest you study on it some more before you do it.

to read the gauges : above zero is positive pres,below zero is a vac. You need to get the system to around the 30" mark and hold it there for at least an hour.Time is not so important as the level of vac. you achieve,but if you dont have a rather pricey micron gauge then you run it for an hour and hope in that time you get it down to 250-500 micron range.The standard gauges dont read in the required vac range , which is lower than 30"
I know I have been rambling here ,but I repeat DONT WASTE YOUR MONEY ON THE H.F. pnuematic vacume pump.you might as well suck it out with a straw.
good luck