2003 GMC Envoy Freon (maybe)?

You won't see refigerant leaking because it vaporizes. There is a special oil that circulates in the system. You might see some residue at the point of a leak. Chrysler systems are very easy to recharge the way you posted because they have a sight glass. When the vapor bubbles disappear, the system is full. GMs have no way to tell how much charge is in the system. Overcharging will lead to poor cooling because the liquid will turn to a vapor too late, on its way to the compressor. The cooling will take place under the hood instead of in the passenger compartment. A severe overcharge can damage the compressor too if liquid sloshes into it. It can only handle a vapor. The only accurate way to know how much refrigerant is in the system is to recover, (drain) the remaining refrigerant out, then pump in the exact measured amount called for.

If you want to try to add one can of refrigerant yourself, since it's not cooling now, one can will not likely cause an overcharge condition. You must be aware though that most professionals will wear a safety shield because in the event of a leak, vaporizing refrigerant can freeze eyeballs and cause frost bite.

The hose that comes with the kit screws onto the low side hose on the truck. It won't fit on the other hose. The engine must be running so the compressor will draw the pressure down in the hose going into it. The can must remain upright so the refrigerant will boil off and enter the system as a vapor. If you turn the can upside down, liquid will enter the system and cause the compressor to lock up. If it takes more than a couple of minutes to enpty the can, place it in a container of hot water. That will really speed things up, but still keep the can upright.

It is common for the charge to slowly be lost after six years, so there might not be a leak that you can find. Be sure to use R-134, not the older R-12.

Caradiodoc

What they will do is connect a pair of gauges to read the pressures while the system is running. The pressures don't mean anything when it is at rest. Some of the refrigerant is liquid and some is vapor. When some vapor is removed, either through a leak over time or by sucking some out with a machine, the pressure in the system goes down. With lower pressure, more of the liquid turns to a vapor and expands which causes the pressure to go right back up. This will continue to happen as long as there is any liquid still in the system.

When the system is running, the compressor causes there to be different pressures in different places. When there isn't enough refrigerant, there isn't enough to compress so the "high side" pressure won't build up high enough and the low side pressure will get drawn down too low. That's the clue.

When your system is so low that it no longer cools, adding one can of refrigerant won't be too much to overcharge it. You don't have to be afraid to do this yourself. Just have respect for the material and don't stick your face near the can. The warning about frost bite and freezing your eyes was only because to most people, that is not common knowledge. If you follow the directions that come with the kit, the job should take less than a half hour and will save you a bunch of bucks. The only problem would be if the system stops working again within a few hours or a few weeks, there is a leak that must be found and repaired.

Caradiodoc