Hydraulics
On a vehicle braking systems, bleed screws, sometimes known as "bleed nipples", are at each wheel of the vehicle to allow bleeding of the braking system. Bleeding air from the brake lines is necessary for correct operation of the hydraulic system (air compresses, reducing maximum pressures) and preventing loss of braking ability after service work is performed on the vehicle's brakes.
Read more about this topic: Bleed Screw