I'm with Neill. Sounds like the dist valve may have moved all the way to one end blocking flow. It happens if you lose pressure on either the fron or the back.
I run a std sized booster with disc and only have 11" vacuum at idle. Brakes work fine. Sometimes they will tell you that if the vac builds up they will work once and then not the second time unless the vac is refreshed but I've never had this happen either so I don't know what to tell you.