Vacuum system is supplied by an engine driven vacuum pump. Power brakes,
and "comfort functions": i.e. engine shut down, climate control and
locks are vacuum powered. Leaks in cars of this age typically occur in
the door lock actuators and can affect the other "comfort" vacuum
A vacuum pump I believe is called MightyVac or Mityvac or similar can be
used to check system or, open the door panel and listen for a hissing
vacuum leak in the lock area. Others may have better and more specific
suggestions on that.
If you like the car be aggressive in fixing the vacuum leaks for the
vacuum system really works well when its leakless.