Air Bleed Operation?

Hi,

The air bleed cyclinder that is mounted on a combi pump. Its a cylinder with a little plastic screw on the top.

I would love to know how it can let air out, but not water??
Plus, how can you test to see if it is working? I have heard that they are not very reliable long term.
When my boiler comes on, lots of bubbly noises can be heard from the pipes before it all settles down. My combi is at the very top of the system, and I guess the air bleed cylinder is its only hope of getting rid of trapped wind...
I would like to learn more about it.

thanks, Alex.

its probably an automatic vent and the cap should be loose or removed, but check the boiler instructions first!

Its just a float valve, when the air is gone the water pushes up and closes it. (for a while)

thanks for the info..