Nissan Repair: PLEASE HELP ME!, nissan sentra, vacuum leak

Question
someone please help me!
i have a 99 nissan sentra gxe, the service engine soon light came on, so i changed the spark plugs, in hope that it was the problem but it still comes on.. and now down near where the brakes are there's a constant hissing noise, as if air is coming through it starts as soon as the car is turned on, and i can feel that the brakes are now getting hard when i press down on it... and now during idel it putts as if it wants to shut off and it has a few times.. can anyone please help me and give me some sort of clue as to what might cause these problems and how i can fix it.. thank you sooo much :)  

Answer
Christina,

You have a vacuum leak and it could be the brake booster or the hose that goes to it.  This excess air will set the check engine light too.  Replace the vacuum hose or booster and then disconnect the battery for a few minutes and the check engine light will be reset too.