A bleed screw is a device used to create a temporary opening in an otherwise closed system, which facilitates the removal of air or another substance from the system by way of pressure and density differences.
On a home radiator unit, the bleed screw can be opened, usually by means of a key, to allow air to escape from the unit. Bleed screws are also found on some pump types fulfilling a similar purpose.
They are most often located at the top of the radiator on the side where the inflow pipe is. The screw itself, usually a hexagonal or square knob, is inside a small round protrusion.
The key looks similar to that used to wind a clock. It is inserted into the protrusion, mates with the bleed screw and turns it. Opening the bleed screw then allows air which has risen to the top of the radiator system (the top of the radiator itself) to escape and new water to take its place. Removing the air and allowing water to displace it makes the radiator work more efficiently since it fills completely whereas before it did not.
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 and preventing loss of braking ability after service work is performed on the vehicle's brakes.
Engine cooling systems can also have bleed screws. They usually takes the form of a bolt with a hole through the middle that is threaded into a hole on the vehicle's cylinder head. This hole goes into the water jacket of the cylinder head. When the bleed screw is loosened, antifreeze is added to the vehicle's cooling system and the increase in fluid pressure displaces air through the passage in the center of the bleed screw. When liquid begins to flow out the passage (meaning all air has been evacuated from the system) the bleed screw is tightened closed. Bleed screws are not common on cars today and are only necessary when design of a vehicle's cooling system results in areas where air can be trapped in the system.