Vented Loops

[markram]

Hi fellow Bavarians...

We are in the process of doing a overhaul of our Volvo Penta D2-55 cooling system on our Bavaria 44', 2002. I have a question about vented loops.
I understand that they are extremely important in order to "cancel out" the syphoning effect of having raw water continue to flow and possible flooding the engine after it has been switched off.
After taking apart the vented loop on our boat (something ancient made of brass!), it turns out that the valve is blocked and has not worked for most probably years! In addition there was a plastic hose attached to the top of the valve leading to an empty bottle to collect I assume any water. 
Despite this, we have never experienced any flooding or any other issues. I reckon our water pump (impeller) is below the water line so in theory we could have experience flooding at any time.
Is anybody familiar with this issue - in the attached photo you can see the vented loop we have - pretty ancient looking piece...

[symphony2]

That is a standard Volvo valve and it should be cleaned periodically as it tends to stick after a while. It should not heed the bottle if it is kept clean so whoever fitted it probably experienced weeping because the little washer was not seating properly. Sometimes people remove the valve and lead a pipe outside into the cockpit so there is a steady stream of water when the engine is running - often called the piddle tube. However that is not practical nor necessary on a Bavaria.

[markram]

Thank you symphony2 - I looked all over the internet and could not find a single reference to this model. So the washer acts like a valve? And under normal circumstances with a clean system no water should  ever flow out?
As I mentioned before, I dont think this has worked in years, so how come we have never experienced any flooding issues? As I said, I think our water pump is below sea level. I am assuming that if it was above sea level then the syphoning action would not take place?

[Yngmar]

I think a stopped impeller pump with a tight fitting impeller isn't letting any water through. So that would stop any siphoning.

[markram]

That's what I also thought Yngmar and obviously that may be what is happening in our case, but I understand that a stopped impeller does not prevent water from passing through.

[allendick]

I had one leak on a 2005 Cruiser in charter and there was water damage to the the starter motor about the same time so, I deduced that was the cause. I was not there at the time and that is my guess from the service records.