I`ve just changed my noisy Central Heating pump with no problems, but
have noticed a brass fitting located at the end of a copper pipe
coming from the pump .

It`s a 1" brass cylinder with a knurled screw end with a red plastic
unscrewable cap on top.

I`m assuming that it is some kind of venting system to remove air from
the CH circuit but how does it work.

Unscrewing the red cap and nothing appears to happen (no gush of air or
water )

I`ve got a normal bleed valve located at the other end of the pump
which can be used for manually bleeding the system using a normal
radiator key.
..

Any suggestions what this other fitting is and how it works?

TIA