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The vacuum brake is not supposed to "hold" vacuum.
So...if it is unable to 'hold' vacuum, obviously this would be the source of a vacuum leak, albiet, a small one. I always, [unlike Charlie] apply a lite coating of moly grease to mine upon assembly in order to assist the 'seal'. Note, there are two different housings too, an aluminum one, and a cast iron unit, which has a bronze bush in it and is my personal preference because of the bush.
I, like Supereal, back the bolt right off, this allows the unit to advance, whilst still imparting a slight drag to the unit, thus preventing an erratic, jerky movement as could happen if one completely removed the brake. My opinion only.