Replaced rear brake pads, now there is a hissing sound coming from brake pedal when i start it , come to complete stop and after i turn car off .what is the hissing or air pressure release sound coming from pedal?

There are 2 things that can use vacuum near the brake pedal. If you have cruise control, there is a safety switch on the pedal to shut off the curise when you apply the brake. This switch may be leaking.
The other part is the brake booster, which is actually in the engine compartment but bolts to the firewall, and your brake pedal mechanism pushes into the booster. You will need to get your head under the dash with the engine running and press on the pedal to locate the leak--on the switch or on the wall.