Happens right after i turn the engine off....

and, when I am on the brakes still rolling and turn the engine off, the get worse also.

Id say its bad. Dammit

  When it works like it should, it should get about 2-3 normal feeling applications once the engine is shut off ( inherent safety feature in case the engine should die unexpectedly while moving with the help of the check valve which locks the vac in the booster for this reason). If it's bad immediately I would say booster is no good.  Unless, the check valve has failed. But if the check valve had failed you most likley wouldn't hear the hissing noise you described.