Engine Cooling
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.
Read more about this topic: Bleed Screw
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