I have a 1998 GMC Jimmy. AC was not cooling. Parts store man said to use r22 which I have for the last two years. Now AC system is not cooling again and I have realized my error in listening to parts store. Can I safely add 134a into my AC system now? What should I do? I am trying to avoid a $100/hr service call.

Your truck's A/C system was designed for R-134a and you should never use R-22.

If you're having to add to the system, there's a leak. Eventually, you'll want to get that fixed or it will end up costing more, just a little bit at a time.

As far as adding to the system, it's best to recharge to the correct capacity, which requires full evacuation of the system as it is now before recharging to the proper amount. If you simply add now, please include some leak-trace dye so finding the leak will be easier.

Definitely a leak if you have to keep charging it. Use 134A and a leak detector you'll see the leak in a few days or if it's that bad in a few minutes.