Parking Brake - Large Vehicles

Large Vehicles

Large vehicles are usually fitted with power operated or power assisted handbrakes. Power assisted handbrakes are usually found on large vans as well as some older heavy vehicles. These operate in the same way as a conventional handbrake, but pulling the lever will operate a valve that allows air or hydraulic pressure or vacuum into a cylinder which applies force to the brake shoes and makes applying the handbrake easier. When releasing the handbrake, the same mechanism also provides assistance to the driver in disengaging the ratchet. Particularly on commercial vehicles with air operated brakes, this has the added benefit of making it much harder or even impossible to release the parking brake when insufficient air pressure is available to operate the brakes. A reservoir or accumulator is usually provided so a limited amount of power assistance is available with the engine off. Power operated handbrakes are fitted to heavy commercial vehicles with air brakes, such as trucks and buses. These usually are spring applied, with air pressure being used to hold the brake off and powerful springs holding the brakes on. In most cases, a small lever in the cab is connected to a valve which can admit air to the parking brake cylinders to release the parking brake, or release the air to apply the brake. On some modern vehicles the valve is operated electrically from a lever or button in the cab. The system is relatively safe since if air pressure is lost the springs will apply the brakes. Also, the system prevents the parking brake being released if there is insufficient air pressure to apply the foot brake. A disadvantage to this system is that if a vehicle requires towing and can not provide its own air supply, an external supply must be provided to allow the parking brake to be released, or the brake shoes must be manually wound off against the springs.