ford cargo air leak?

Hi everyone,
I have an air leak as soon as I release the brakes to drive off. Its gushing out of the bottom of a component which is a junction box of air lines at the rear, bolted to the chassis beside a small metal bottle whichis also something to do with the air.
The square junction box has a rubber disc at the bottom of it which is perished. This is where the air is coming out of. The air loss just keeps up with the air build up, so I can still drive it, but mess with the hand brake too much (not much!) and the hand brake goes on. Does anyone know the name of this box, and come to think of it, what is the bottle/small tank next to it, and what is their functions, and can I service/ repair my leak? Thanks in advance.