Four-wheel drum brake models have a pressure differential switch to alert the driver when one of the two brake circuits loses pressure. On disc-drum models, this switch is combined with a pressure metering valve to limit pressure to the rear brakes and prevent rear wheel locking. Both types are located in the engine compartment, usually on the frame rail. These valves reset themselves automatically after the malfunction is corrected and the brakes used. They do not interfere with normal system bleeding; however, if pressure bleeding is attempted on disc brake cars, a spring clip must be used to hold the metering valve stem out.