2002 Toyota Sequoia brakes hiss

want to know why when i press on break pedal i hear hissing sound.

I could be wrong but you may have a little bit of air in the system. Take the vehicule to an independant mechanic for an inspection of the brake system.

My first thought is you have a leak in your power booster.

If you disconnect and plug the vacuum hose feeding the brake booster unit, do you still get the hiss?