Bogie Pivot pin breather.

[Lynn Eades]

Alistair, it should have a ball and spring. Maybe the spring is broken or rusty?
or the ball has rusted at the "out" end and no longer seals?
Anyhow, You could put a grease nipple in there but would need to remove the ball and spring first. Grease must be able to exit there.
I say this because I have NOS Australian carrier ones. They come in two types one is as yours should be with check valve, and the other is an open vent. They are made of steel and have a UNF thread (the only differences)
Basically you grease the grease nipple until grease comes out the check valve. Do that regularly and after you've been through water and it will out last you.