Most air brake systems use pancake air pressure switches. They will look something like this Truck Trailer Spring Brake Light Switch Low Air
Keep in mind they come in different pressures and do different things but you likely have several in your braking system. One issue I have seen many times over the years is they get covered with crude and short out across the tops of the terminals. Often when driving in the rain. I take some cleaner like brake cleaner and clean them then spray a coat of sealer over them. Like I said there are likely several on your air system so it can take a while to find out which one is the problem.