As others have mentioned, you need a set of gauges, and a vacuum pump.
Here's an image, link and pricing for the gauges from Harborfreight:
$49.99
http://www.harborfreight.com/catalogsearch/result/?category=&q=134+gauges
And also a vacuum pump. The first shown is a one that you use with an air line from your shop compressor, and the second is an electric.
$12.99,
http://www.harborfreight.com/catalogsearch/result/?category=&q=vacuum+pump+134
$79.99,
http://www.harborfreight.com/25-cfm-vacuum-pump-98076.html
I would suggest checking the orifice tube for debris. It's located in the lower tube of the condensor at the threaded line connections near the battery. This would clue you in to knowing if there is any debris in the system.
$1.27,
http://www.ackits.com/merchant.mvc?...en=PROD&Store_Code=01&Product_Code=31-50006&Category_Code=Chevrolet96caprice5-7
If this looks like it has any particles/debris on it, especially in the screen area, I'd suggest flushing the system (Condensor, evaporator core, and lines). This is the proper way to flush the heat exchangers:
The Evaporator's inlet is at the bottom and therefore only one flush in the back flush direction (
top to bottom).
The condenser's inlet is at the top and as with all heat exchanger flushing, this must be back flushed first (
bottom to top).
I used the cheap flush stuff available from Advanced, and although it worked, I was happier using a fluid gun and acetone. The stuff from Advanced didn't evaporate fast, much at all after I flushed with it, and used compressed air to follow it up. Acetone will flush the system of contaminents (and water!), and evaporate quickly.
The next thing to do is replace the O-rings and sealing washers @ the compressor. O-rings can be bought from Ackits.com as well, and here's an image, pricing, and a link:
$10.51,
http://www.ackits.com/merchant.mvc?...reen=PROD&Store_Code=01&Product_Code=CP3016&Category_Code=Chevrolet96caprice5-7
The last thing is the sealing washers at the back of the compressor. I believe you'll need a 17MM socket to get that flange nut off. These are ~$10 at the stealership
I like the typical electric operated vacuump pump, as opposed to the super cheapie myself. I like to draw a vacuum on the system for at least an hour. Using the gauges & pump you should be able to pull 30" of vacuum without a problem. Then, I pull the plug on the vacuum pump, and leave it connected for another hour to ensure the system is holding a vacuum.
Then charge the system thru the gauges (R-134 gets connected to the center {yellow} hose, and thru the low pressure (blue) hose.
The last thing I'd highly suggest on replacing would be the accumulator (receiver/dryer). They're cheap, and it'll ensure you don't have moisture in the system
$15.60,
http://www.ackits.com/merchant.mvc?...en=PROD&Store_Code=01&Product_Code=37-23289&Category_Code=Chevrolet96caprice5-7
Good luck.