I use the hose kit with gauge. You can re-use it with a new can of refrigerant. Usually if the coolant is low, the compressor will short cycle. With AC on, you can see the gauge go up for 5-15 seconds then it drops off. If it is really low, it won't come on at all. I usually add enough refrigerant one can at a time to get it running smooth. No more. too much can damage the compressor. If it takes more than one can, it has a pretty good leak.