How to replace my AC system in my 1991 C1500 Silverado 5.7.

If it's just the compressor, buy one at a junk yard or on ebay that will fit your truck. It's a pretty easy install, you have lots of room on that truck. Pretty straight forward. I'm assuming the refrigerant leaked out, so you can do no additional harm to the ozone. Check to make sure though, on the low pressure ref. Line, the fat one, there is what looks like a valve stem. Open that cap and push the valve in for a second. If you hear pressure, you'll need to go to a shop to have the freon captured, it's the law, at least where I am anyway. If you have no pressure, swap out that pump. You can get the new refrigerant at Walmart or Autozone, just follow the instructions putting it in, don't worry about old and new freon, it will work just fine with the new stuff. The only thing you may need is the conversion kit which is a couple bucks and just a fitting to accept a larger filler. Remove this fitting when you're not filling, they tend to leak. Make sure the car is running and AC blowing full cold. It will seem like it's not doing anything until you get enough in thetre to keep the compressor on, It shuts off if the pressure gets too low. Also, the cans get cold while filling, so you may need to stop for a while and let it heat up, because a cold can doesn't have enough pressure. Let me know how it works out. It's not a big deal if you have an adequate set of tools.

Dude, spend 10-20 bucks, give this a shot. Go buy a bottle of R134A or whatever from Walmart, look for something with a bottle and a gauge on it, they're disposable. You might need the valve kit, like I said are cheap. Follow the instructions, I bet you will have AC for a year, and maybe forever. Get the stuff with leak seal in it. I'll walk you through it over the phone if you really need it. I keep this stuff in my car all the time, and amaze people all the time as I fix thier AC in 10 minutes for $20 of freon. The fact that you have cold and then not and then have it and then not tells me you have something that is worth attempting to fix the cheap way.

Walmart or autozone, it screws on to the current adapter. Just make sure to get in the high range of the green zone. The can will get cold like I said, go ahead and get another one, sometimes it takes two cans to get a good seal. Remember, the system only accepts freon when the pump is running, so you will know that you're on the right track when you hear it stay on rather than on and off. Good luck! Oh and take tha adapter off when you're done and put it in a safe place like the end of the freon can or an ashtray. They can cause leaks themselves.

You are correct, just top it off, get into the high green range.