my servo has no tube on it?

hello again, my servo unit on my 109 series 3 2.25 diesel has a servo unit but no tube connected to it?
to be honest i dont know what the servo does, but someone told me i helps the breaks and there should be a tube going to the engine somewhere but i dont know where this tube goes or why i dont have one?
any help gratefully received.

On later models tube goes to vacume pump on block,but on series with servo it went to the inlet manifold where a butterfly valve is linked in to close when you take your foot off.This then develops a low pressure in said inlet manifold as the engine runs down.