Forespar Performance Products 841 Vent Cap Assembly MF. Vented loops are commonly used in the marine toilet system and engine cooling system. The simple valve system incorporated in the top of the loop allows air to enter the line when not in use yet effectively seals as water or other fluid passes through. Molded in Marelon neither vent nor loop will corrode. The duckbill one way valve is designed to seal properly if particulate matter is present in fluids. Specifications for Forespar Performance Products MF 841 Vent Cap Assembly: Manufacturer: Forespar Performance Products Color: Black Package Contents: Forespar Performance Products MF 841 Vent Cap Assembly Forespar Performance Products 903002: 841 Vent Cap Assembly MF