You'll have to check the vacuum lines for leaks... the vacuum source is at the mechanical vacuum pump on the front
of the engine. Follow the big hose towards the brake booster... you should see a small vacuum hose that "T's" into the
big hose... following the little hose should lead you to a "vacuum reservoir & check valve" and another "T" fitting that
feeds a vacuum hose to the Heater Box Assembly & Control Levers.

Sounds like you have a disconnected hose, broken fitting or a check valve that is not holding vacuum.

Thomas