Rear axle breather

Information please on how the rear axle breather works. On my 2001 model there is a capped tube about 30mm long protruding from the top of the nearside axle tube. When I remove the cap and insert a piece of wire, it hits an obstruction blocking the tube at the bottom. I cannot see how this can allow the axle to breath.

What am I missing here?

Thanks in advance.

Inside should have spring with a flat round rubber
The cap it's self should slide up and down on the spring
SO if axle is warm and you enter cold water.Axle will contract and suck in air
Spring will compress the rubber should seal it

IN reality it's so old it won't work
Get a 1/8 bsp tap pull the cap spring and rubber put them in bin
Buy a pneumatic 6mm push fit elbow with 1/8 treads and make a breather

Jack