brake booster 02 caravan

My friend has 02 caravan SE 3.3l exactly like mine except the color. His van will not move when engine is running(even in nuetral). A friend of ours suggested that the brake booster has failed and overridden the master cylinder locking the brakes. With the engine off, in nuetral, it rolls.

Isolate the brake booster from the engine.

1) Plug, or put a turnicate on the port were the brake booster connects to the intake for vacuum.
2) Start the engine.

If engine is ok, you got a bad brake booster.