Possibly the most important minor component of your entire kit, without a properly working pressure switch, your kit will not work. While there isn’t really any maintenance that needs to be done (other than draining the tank on a regular basis), wiring your pressure switch is vital to getting your system to work.
The most important thing to note is that some pressure switches are sealed and some are not. While all pressure switches can be on systems installed outside the vehicle, some pressure switches are more weather resistant than others. If you plan on getting your ride wet or dirty on a regular basis, you want to get a sealed pressure switch, such as one that comes with leads.