assisted parking brakes. Power assisted parking brakes are usually found on large vans as well as some older heavy vehicles. These operate in the same way as a conventional parking brakes, but pulling the lever operates a valve that allows air or hydraulic pressure or vacuum into a cylinder which applies force to the brake shoes and makes applying the parking brake easier. When releasing the parking brake, 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 parking brakes are fitted to heavy commercial vehicles with air brakes, such as trucks and buses. These are usually 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 cannot 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. Railroad hand brakes Virtually all railroad rolling stock is equipped with manually operated mechanical hand brake devices that set and release the brakes. Most of these involve a chain linked to the brake rigging, most often at the brake cylinder, that when tightened pull the piston out against the releasing springs, thus applying the brakes on the car (if there is only one brake cylinder per car) or bogie (if there is more than one cylinder per car). Newer locomotives have electric systems that simply place an electric