A bleed valve, or pet cock valve as it is more commonly called, is used to remove air and liquid from a vehicle's radiator or engine block. Resembling a "T," the bleed valve has two wings that can be turned by hand. The two wings are situated on both sides of a hollow tube that complete the valve assembly. If unable to be turned by hand, a small pair of pliers or an adjustable wrench will usually persuade a stuck valve to turn and drain the system.
The importance of a bleed valve is two-fold. First, the cooling system is easily bled, allowing any trapped air in the system to escape. Air trapped within the system will cause the engine to run hot, as the coolant will not flow through the system properly. Hot spots, which are caused by the coolant not completely filling the entire space within the engine, can lead to gasket failure, water pump failure and engine failure.
Secondly, fluids are easily drained from the vehicle through the removal of the bleed valve. This is the only method for draining the cooling system short of removing radiator hoses. When storing a vehicle through cold winter months, draining the coolant down can prevent freezing and cracking the engine block. As the coolant level is lowered in the cooling system, room is made for expansion of any ice that may form. Expansion is the culprit behind broken engine parts from ice; as the ice expands and has no room to grow, the engine block will crack in order to create room for the ice.
When testing anti-freeze levels in a vehicle's cooling system, many mechanics will collect and test coolant from the vehicle's engine block. By draining a small amount of coolant from the valve located on the engine block, the mechanic is able to verify the anti-freeze level in the engine and not only the radiator. This test shows that the coolant is protected and circulated through the entire cooling system. Testing the radiator only does not show that the coolant has circulated through the entire system, thereby achieving total protection from freezing.
Most vehicles also feature a bleed valve into the braking system. This allows the brake system to be bled easily by removing large quantities of air. This is especially important when removing or replacing the brake's master cylinder. By placing a bleed valve in the brake system, the mechanic is able to save time bleeding the air from the system without having to pump the brake pedal and loosening the brake line to allow trapped air to escape.