[blykins]

OK....I see how this works now. You do need vacuum on each side of the diaphragm. The diaphragm is encased in a vacuum sealed area....when you apply the brake, the valve opens, allowing atmospheric pressure in...giving you vacuum on one side of the diaphragm and then atmospheric pressure on the other. However, when you release the brake, that valve closes, allowing the vacuum to come to both sides.

If you didn't have the diaphragm encased in a vacuum at all times, your brakes would continually have pressure on them. You need a point there where there is nothing acting upon that diaphgram.