You CAN NOT properly service an A/C system unless you are monitoring BOTH high and low pressure in the system…if you overcharge the system, the high side will respond first, with pressures quickly climbing way above normal…With the $20 recharge kits, you will be unaware of this and can easily damage yourself and your A/C system…If you INSIST on DIY A/C servicing, at least buy a SET of gauges so you can see what you are doing…