Anyone use those cans of r134 to refresh a/c systems? They're fairly idiot proof. They come with an attached gauge, they only fit the low side port, etc. My question is, is the gauge a "smart gauge" meaning if it indicates a properly charged system will it prevent you from adding refrigerant to the point where you might risk overcharging the system. Both of my cars started off in the middle of the green so to speak but after several seconds of hitting the trigger even though the gauge didn't budge the a/c is noticeably cooler. Does a little go a long way?