Not exactly sure of the name but think its an anti siphon valve.
Anyway, I have just changed the impeller and now it appears to be dripping down the drain pipe at a rate of about 6 drips a minute. I don't know if its just coincidence that the impeller change had anything to do with it or not.
Can someone please explain what this device actually does and how I can stop it dripping?
Many thanks.
Probably just needs cleaning. There is a little rubber washer inside and a bit of dirt there will stop it sealing properly while the engine is running.
I had the same problem with the same Anti-Siphoning Valve for a couple years off and on. I stopped it for short times by, as Symphony said, cleaning.
This spring I stopped it for good by replacing it with a new one. 300 hours this summer and not a drop.
Be careful where its dripping. My drip was travelling down one of the pipes then dropping on the Alternator with terminal effect. I ended up replacing the alternator.
Tip: If you have a drip hose like that on anything, put a plastic bottle on the open end (re-openable zip-ties work to attach them) instead of just letting it end in the bilge (or wherever). This way not only everything gets caught, but you also see at a glance what exactly has been dripping.
When the syphon breaker fails there is a risk of water finding it's way into the engine so I decided to eliminate this risk by removing the jiggle valve and connecting the pipe to the top of the water lock. Anti syphon works every time and the water loss in the cooling system is negligible.