Most all cars have a check valve from intake manifold to booster to prevent vacuum loss from booster when engine isn't creating vacuum.
When the booster is not being supplied with vacuum, you get about 2-3 uses of power assisted brakes before the pedal becomes much harder to push.
So, a few things could be happening:
-everything is mechanically OK, but you are using brakes whilst the engine is not producing vacuum. Ideally you should generally be off the accelerator when applying brakes, which should cause the engine to go to vacuum. So, are you applying brakes at same time as accelerator?
-the vacuum check valve could have failed allowing booster vacuum to bleed off.
-is there an engine condition causing the manifold to not go to vacuum when you release accelerator? Boost leak, maybe?
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