or pull of a button rather than a mechanical lever or pedal in the cabin. A more complex unit (first seen on the 2003 Audi A8) uses a computer-controlled motor attached to each of the two rear brake calipers referred to as the Motor on Caliper (MoC) system. Many car manufacturers such as Jaguar, Land Rover, BMW, Renault, Subaru and Volkswagen sell models whereby the parking brake automatically engages when the vehicle is stopped and is released when the gas pedal is pressed, eliminating the need for the driver to operate a button. An extension of this system, known as hill-hold or hill-assist, prevents the vehicle from rolling back when moving-off on an uphill gradient. Types of brakes In vehicles with rear disc brakes, the parking brake either actuates the disc calipers (with much less force) or a small drum brake housed within the hub assembly (the inner circumference of the disc is often used instead of a separate drum). This secondary drum parking brake is often referred to as a banksia brake. It is usually used in conjunction with rear disc brakes employing multi-piston calipers, as mechanically actuating these calipers is more difficult than single piston calipers. Another less common setup for rear discs is the use of a separate, smaller, cable actuated caliper (such as the Wilwood MC4), which is used exclusively for the parking brake. This is sometimes used as an alternative to a separate drum parking brake when multi-piston main calipers are used. Hudson automobiles used an unusual hybrid hydraulic-mechanical dual-brake system which operated the rear brakes through the otherwise conventional mechanical parking brake system when a failure of the hydraulic system allowed the pedal to travel beyond its normal limit. A number of production vehicles, light and medium duty trucks, and motor homes have been made with a separate drum brake on the driveline; called a transmission brake. This has an advantage of being completely independent of other braking systems. This is effective when there are multiple driving axles, all driven wheels are braked at once. A line lock is a temporary parking brake that makes use of the vehicles standard hydraulic brakes. They are often used for off road conditions or when stopping on steep grades is required. By trapping hydraulic pressure in the brake lines, all four wheels can be locked. Large vehicles Large vehicles are usually fitted with power operated or power