The mechanical stop light switch is operated by contact with the brake pedal or with a bracket attached to the pedal. The hydraulic switch is operated by hydraulic pressure developed in the master cylinder. In both types, the circuit through the switch is open when the brake pedal is released. When the brakes are applied, the circuit through the switch closes and causes the stop lights to come on. Hydraulic brake light switches are not commonly used today because they provided one more area for possible fluid leaks.