Breather Filters are a component of the positive crankcase ventilation (PCV) system. The PCV system's function is to rid the engine of gases produced from a overly-rich fuel mixture, improper fuel/air settings, and worn piston rings of cylinders. The PCV system generally consists of a PCV valve, a vacuum line, and the breather filter.

The blow-by vapors are drawn from the crankcase through the vacuum line and into the intake manifold, where they are mixed with the fuel/air mixture from the carburetor or the fuel injection system, and then burned in the cylinders. To make up for this loss of pressure in the crankcase, fresh air flows through the air cleaner, breather, and PCV valve into the crankcase.