Weird brake pipe thing

I've just taken the front arch (drivers side) off my Kombi, a 73 1700 which has got the vacuum brakes. There's a weird plastic pipe that's taken off just near the brake servo plumbed into the front arch (or there was, 'cos there isn't any more...)

Does this have any function other than to suck rust into the servo - is it supposedly a source of drier air?

Cheers
Si

Proudly throwing money at a pile of rust.