CSF Radiator Cap Honda | Acura

When the engine cools and pressure in the system drops, the radiator pressure cap allows the excess coolant in the overflow tank to return to the system with a second spring-loaded valve. This second valve responds to the vacuum created in the system from the drop in pressure and pulls the coolant out of the overflow tank. Not only does this recycle the excess coolant to prevent air pockets in the radiator, but it also prevents the softer components of the cooling system, such as the hoses, from becoming crushed by the external air pressure.