The vacuum for the brakes is created via the engine, via a take-off from the
inlet manifold.  With the engine running it is near instantaneous.
Disconnect the hose to the brake servo and with the help of a friend do:

1) put you fingure over the end of the hose
2) get your assitant to start the engine.
3) you should quickly feel the "suck" on you finger
4) gently start to pry/remove your finger from the vacuum hose.  You should
heat the air rush and the engine revs with change.
5) with your finger totally off the end of the hose the engine may stall,
rev fast or even over rev (so ensure your assistant is ready to quickly turn
the ignition off.

If the engine is creating  vaccum and there is no obvious loss of air,
indicated by hissing noises somewhere between the end of the hose where your
finger is shutting it off and where the hose attaches to the engine/inlet
manifold, then the problem lies either in the one way inlet valve to the
brake servo unit or a perforated diaphram in the servo unit.  If it is the
little one way valve then this is easily sorted.  Otherwise you need to
replace the brake servo unit.

Nick /////