I have a '97 Blazer. This is rather long so bear with me. When I bought it last month the previous owner said the air conditioning doesn't work. Last Friday I was able to borrow an air conditioner manifold gauge set and a vacuum pump. When I hooked up the gauges there was 0 psi in the system. I applied a vacuum and in less than a minute I was able to draw 30" vacuum. I left the vacuum pump on for an hour to evacuate the system, the whole time it maintained 30". I then disconnected all the test gauges and about 2 hours latter reconnected the gauge set without the vacuum pump. The gauges indicated that the sytem held 30" vacuum for at least 2 hours.I then charged the system adding oil and 30oz 134a. System ran great, very cold, compressor cycling on-off as it would in a fully charged system. Now a little more than a week later the compresser stays on for about 4 seconds and then cylces off which I attribute to low pressure in the systemso I obviously have a leak. My questions are, if there was a leak in the system, why did the system hold vacuum for 2 hours a week earlier? My only thought is that I did the vacum test with the engine off and the compresser not running. Since the compresser has a pully, there must be a seal between it and the compresser shaft and that as the compressor turns, the worn seal is allowing the R134a to leak out. Is ther anyway to confirm this? I was going to apply vacuum again to the system, start the engine and jumper out the low pressure switch to allow the compressor to turn hoping that if it was a bad compresser seal, it would not hold vacuum. Also, the leak detection gas the parts store sell, can I use that or do I need a special light? I have very limited knowledge of AC systems. All help and suggestions are appreciated.
Thanks
Thanks

 

you can use the dye kits they have at part stores it comes with a light and glasses plus of course the dye which you will need to do it in like a garage to see it better but try that first before replacing parts

 

It's quite possible that it is the front compressor shaft seal. An indication would be an oil stain on the underside of the hood from the oil being thrown off the clutch.
Also check the a/c line connectors at the condensor and at the drier/reciever. If you have leaks at the a/c line connections there is usually a black oily stain there where the oil has mixed with the outside dust.
If you can't find any leaks that way then as thegr81 suggests, recharge the system, adding dye and have it checked with a black light kit (black light and yellow sunglasses)
Most common leak is probably the compressor shaft clutch seal, the seals tend to dry and harden if the system is low on oil or the system doesn't get used often enough to circulate the oil.