It is a small valve often mounted at the top of a vented loop, designed to introduce air when a suction  is applied.

It prevents a reverse siphon that would cause seawater to enter any device mounted below the waterline (engine, head, or tank for example) when plumbed to an under water through hull fitting.

When fluid is pumped overboard, the pressure causes a tiny, rubber “duck bill” valve to close. When the pump stops, if fluid tries to move back into the boat, the anti siphon valve opens, introducing air that breaks the siphon and prevents flooding.

It must be mounted high, in a location that is never below the waterline even when the boat is healed and underway. It is typically up under a side deck or high in the engine compathment. Follow the discharge hose in question from either end to locate its exact location.

Yes, over time the rubber valve will get hard. If it isn’t super flexible, it will no longer open to break the siphon and will stop protecting your boat. Like the rubber components in a bilge pump, these inexpensive, safety related items should be replaced every two years. It’s a 2 minute job and protects your boat and engine from flooding.

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Just un-screw the cap, remove the rubber valve, and replace it with the new one. Make sure the “duck bill” is pointed down when finished.