If it's an air brake bus the switches are located under the dash on the brake pedal valve. They are like a quick disconnect air fitting. First purge all of the air from the system and then locate the switch, depress the brass ring on the fitting and simply pull the switch from the fitting. Pretty simple fix. If the bus has cruise control there will be multiple switches on the brake valve that look identical. You will need to disconect the electrical connector of each switch and observe which causes the brake lights to go out and that is the one you are looking for.