S&S Cycle Crankcase Breather Reed Valve946242785 Product Description New style breather is not dependent on the rotation of the crankshaft to make it open and close. Instead, reeds open when pressure in the crankcase becomes greater than pressure in the cam chest, and close when pressure in the cam chest is equal to or greater than pressure in the crankcase By opening and closing "on demand," the reed valves automatically compensate for lag time associated with accelerating the mass of air