I though some may be interested in this. I just got an old run around car to save some miles and gas on my truck. I took this lil car into a local mechanic to have the AC charged and checked as it was blowing cold but barely.
Anyway he did the proper steps, evacuated the old stuff and held a vacuum, found no leaks, and recharged with the proper oil and gas. System about froze me out! I was going to just try a can of 134a at the parts store, but that is NOT the correct way to do that. You really should evacuate to get all the old air water stuff out, and have a proper set of manifold gauges and the proper oil and proper gas level.
I have been thinking my ac was a tad weak in my F150 super crew. I did the insulation and heat tape on my line going to the evaporator. Helped but not all the way there.
When that car was in the shop I was talking with the mechanic about the new R134 a systems. He misses the R12 days... He said with that 134a the molecules are so small and pressures are much much higher that all the systems "leak" very slowly on most but still leak.
He was running a special at $50 for the service plus the gas. So at less then $100 I got my system evacuated, checked over, and recharged. My system was 8oz low! I asked him about that and he said that is very normal for a truck that is 8 yrs old. He stated that on the average he sees that the 134a will seep out about 1oz a year. It now blows so cold that I had to turn it down a fan speed!
Just for an example. Yesterday it was 98F here with a heat index of 111F Hot and humid. 2 weeks before it was about the same. 94F and 104 heat index. I noticed this is when my system would struggle. Not anymore! I had to turn it down to speed 2 on the fan. It was very cold. and that was with more heat and humidity!
My wife even asked me to turn it down yesterday as well. She said it was freezing her out!
So for those with a weak AC I would highly suggest getting your system checked out. I do not recommend just putting in more gas as you have no clue what the level is and oil and what else is in the system. Plus if you over pressurize or over fill you could cause big problems and big costs. My guy charged me under $100 bucks. A can of R134a with a cheepo gauge was $45 bucks alone. Just a local shop with a trusted guy. Call around get references you will find one in your area.