Our duckbill valves are intended for use with the VG4 J-Series Vacuum Generator and M-Pump or large in-line check valves (308300011). 2″ Duckbill Valves (4pack) are the number one maintenance item. When diagnosing a vacuflush system we always suggest starting with a “fresh” set of duckbill valves. If your vacuum pump is no longer shutting off, it’s time to change the duckbills. All VG4 Vacuum Generators and M-Pumps contain 4 Valves. Check out our youtube VIDEO on how to change the Duckbill Valves on a VG4. While you have the pump body off of the tank, many customers choose to change the pump-to-tank O-ring. Your owner’s manual or service records may indicate part number/ SKU 3853112308. The 385311208 is a 2 pack of 2″ Valves, while the 385311581 is a 4 Pack of 2″ Duckbill valves. The 4 pack and 2 pack are so close in the price we only offer the 4 pack.