On 26-Mar-18 8:42 PM, Harry Bloomfield wrote:
on 26/03/2018, harry supposed :
They have a little float in them. If air appears, it falls ans opens a
valve in the top.

You can* dismantle and clean any crud out and clean the valve seat and
needle.* (They can stick a bit or get something on the valve seat.)

I have come across another type, where a material gets wet, causing it
to swell and thus acts as a valve.

That is the type I know. They use the same type of fibre that is used
for sealing washers in water systems. They seem to be very reliable.

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Colin Bignell