Air exhaust fittings are used to equalize pressure in a customer water cooling system with the air pressure outside. They come in two forms: automatic and manual. Automatic air exhaust valves are normally installed in the top of a reservoir or some other spot in a custom loop where air gathers. By using a spring-loaded check valve inside the fitting, any excess pressure inside the system is allowed to escape, preventing damage or leaks. Manual air exhaust valves are effectively the opposite of the automatic variety, with the spring-loaded valve opening inward instead of out. Used as needed to simply open the water loop to the outside air, they are helpful when bleeding a new water loop, providing a spot to purge the air from the system.