I need to ensure I've got all the air out of my closed pressurised
system in order to resolve/eliminate another issue. Pressure is
around 0.8bar when I start, down to about 0.5bar when I've finished
and I then repressurise.

The bleed screws are those little square ones at the back (not side)
of each radiator so when they squirt out they hit the wallpaper.

The property is a bungalow so all radiators at ground level but the
pipes go up and over via the attic.

Anyhow most of the radiators instantly let out a silent stream of
water.

A couple sputter, water comes out but with a sputtering sound and/or a
whistling sound and this is where I'm confused. It seems no matter
how much I let out (I'm talking cup fulls not gallons) they still seem
to sputter. They sputtered yesterday, today and I expect they will
tomorrow.

Is this air in the system or is it just the shape of the screw?

Am I correct in assuming that air cannot be introduced into the system
unless there is serious corrosion or the like going on?

How much oxygen am I reintroducing repressurising the system?

--
AnthonyL