Snoop around here first:
http://www.autoacforum.com/
If you have bubbles in the sight glass of the reciever/drier, you have a leak somewhere.
A 609 Certification doesn't teach you a thing, other than the harm that CFC's do to the ozone, and R-12 regulations.
If you don't know what you are doing, you can seriously hurt yourself. A can of r-12 is potentially as dangerous as half a stick of dynamite. Do yourself a favor and buy a book. The Haynes A/C manual does a pretty good job actually.
To do it right, you'll need a real set of guages, and a vacuum pump (the air powered ones aren't all that great, and won't do the job right). I know that people have just dumped some in, and it cooled O.K. without buying the right tools, but I'm sure you can rebuild an engine with a pair of pliers too. I like having the right tools for the job.
Legally, you can't just dump in a can of R-12. You have to recover the old R-12, test the system for leaks, repair the leaks, and then fill with new, or recycled (not reclaimed) R-12. If you get caught not doing it by the book, it's a $15,000 fine, and $10,000 of that goes to the person that turned you in.
It isn't hard to retrofit an XJ to R-134A. They actually cool pretty good with it too.
You need to replace all of the O-rings, flush the system to remove ALL of the old oil (even in the compressor), a new reciever drier, install a high pressure cutoff switch, new R-134a fittings, vacuum it down to remove all of the moisture, add the new R-134a specific oil, and THEN recharge it. You are looking for a high side reading of 2 - 2.5 times the ambient temperature in PSI. If it's 90 degrees outside, you are looking for a high side reading of 180 - 225 PSI (90 X 2 = 180, 90 X 2.5 = 225 PSI) Get the high side up to 180 PSI, stick a thermometer in the center vent, turn the blower on high, let the temp equalize and adjust the pressure to get the coldest reading. Let the temp equalize after each adjustment. Don't go over 2.5 times ambient, the compressor won't last long. They weren't designed to run at really high pressures. Mine gets to around 45 degrees at 90 degrees outside temp. The heater valves don't seal well on XJ's either, so disconnecting the heater hoses to the heater core helps quite a bit too. Older XJ's use an "H" valve instead of an orifice tube. The H valve relies "mostly" on temperature so it doesn't really know if it's R-12, or R134A, and it's pretty forgiving on over/under charging of the system.
Hope it helps.