I also hate those kits, when I have to do A/C service at work it makes things a mess. The proper way to fill the system is to have all of the old refrigerant recovered, and the system pulled into a vacuum. This removes all of the air, which contains moisture. Then, a measured amount of refrigerant is put into the system. This is the only way to do it.

If you shoot refrigerant into the system without having it vac'd down, there is still air in there. It will cause poor cooling, high pressures, and eventual corrosion damage from the trapped moisture. You need the proper equipment to do the job.

Sorry if I sound like a hardass about this, but there is a correct way to do it. I'm 609 certified along with state of Wisconsin certified which is even stricter than federal. It's in the same group as California.

Refrigerant is more dangerous than people think, so please be careful when handling it.