44610-22730 is a 9inch signle product,for Toyota CRESIDA RX80/81,GX81.The vacuum booster or vacuum servo is used in most modern hydraulic brake systems which contain four wheels due to reliability, low maintenance, and adequate brake assist for most applications. Alternative means of brake assist exist for special purposes or performance such as assist powered using the vehicles power steering pressure or a dedicated fluid pump as found in Teves ABS systems. The vacuum booster is attached between the master cylinder and the brake pedal and assists the braking force applied by the driver. These units consist of a hollow housing with a movable rubber diaphragm across the center, creating two chambers. When attached to the intake manifold of the engine or the vacuum pump, the pressure in both chambers of the unit is lowered. The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed. A return spring keeps the diaphragm in the starting position until the brake pedal is applied.