Sometimes, not always, I hear a hissing sound behind my break pedal, especially right after I turn off the engine. I can make the hissing go away by tapping the break pedal a bit. What kind of a leak do I have and should I be alarmed?

 

The diaphragm inside of the brake booster has a leak. Worst case scenario is you will lose power brakes, leaving you with brakes that require a lot of foot pressure to make the vehicle stop.

 

and how is this fixed? Is it a DIY project?

 

It's not difficult knowledge and tools wise, but it is a PITA because you have to be able to reach up under the dash to remove 4 locknuts and disconnect the linkage. Under the hood you have to unbolt the master cylinder and pull it away from the booster for clearance.