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"Should I have gotten a vacuum and pulled three lbs?"
The answer is yes, maybe, no.
If your system was empty enough to have allowed air (and moisture) to enter, then you really must vacuum the system. This removes the air and will evaporate any (water) condensation and remove it as vapor.
However, if your system merely had a leak and still had a slight positive pressure of Freon, then you can often finish the repair and simply recharge it. But pulling a vacuum is "going by the book."
Normally you pull a vacuum that's measured and expressed as inches of mercury, and a decent pump should pull about 29" Hg or a bit more. This is almost full atmospheric pressure, which would be 14.7 psi. However, vacuum is rarely measured or expressed as pounds or psi, whereas pressure almost always is.
"And where do I get such a vacuum?"
You need a vacuum pump that's suitable for pulling a really good vacuum. One clever pump uses compressed air to create the vacuum -- JC Whitney and Harbor Freight sell them for around $20 (no moving parts). The disadvantge is that they consume a lot of compressed air.
A GAST vane type pump is an excellent laboratory-grade pump, but it will set you back a few bucks. I sometimes see them on eBay. Other makes of vane-type vacuum pumps would probably suit you, but they're hundreds of dollars. That's too much for the average car enthusiast.
"And how does it work?"
It sucks the air out of your A/C plumbing. You connect the pump to the system via the hoses on your gauge set. This means your pump must have the proper fitting for the hose.
"...just added a couple of cans...seems to work fine..."
I suspect you fall in the category where your system never completely emptied, so was not contaminated.
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Don Foster (near Cape Cod, MA)
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