Originally Posted by MDBones
I'm wondering if your Jeep had the brake booster recall done prior to you purchasing it? A number of people here reported that their dealers also screwed up the master cylinder as part of this repair.
Here's the recall and repair procedure... http://wk2jeeps.com/tsb/rc_P1414.pdf
What he said.
If hasn't been messed with, I would guess the hiss is either a leaky vacuum booster or bad vacuum hose to the booster. Might be able to tell by opening the hood.
? Temperature when the brakes failed ?
I'd be somewhat inclined to have a chat with the selling dealer immediately.