That is an automatic bleeder valve - they are subject to becoming less automatic to non-functional over time, and yours are well into the "heavily corroded old age" stage of their lives, or at least the one in the picture is.
If you don't get air when you actuate the valve, this location is bled.
When working properly, any air is collected in the chamber, and once enough air has collected a float drops and causes the air to be vented. The valve cap is normally left somewhat loose so that air can vent, until they start leaking and you have to disable the automatic part of their function by cranking the valve cap down until it seals (normally has a rubber seal inside the cap for that purpose.)
They still work as an air collection point that way, but you have to unscrew the cap and let the air out, then reseal it so they don't leak water at that point.
Here is a cutaway view (from caleffi.com, never heard of them, no affiliation) showing their version of the mechanism.