My 2003 ford expedition just started a hissing noise near the brake pedal that stops when I press on the brake 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.

your brake booster is leaking internally. this is a one piece, non serviceable component, and the entire assembly must be replaced. good luck!

A vacuum leak. Look & listen for hissing noise where rubber or nylon lines come through firewall under dash near brake pedal. Also look under hood at large hose going to back of brake master cylinder, power brake booster.

I have had the same problem with my 05 Escape and it was actually the rubber boot behind the brake pedal that was torn if I started the car and pressed down on the brake with my hand and let it up I could feel the air coming off of the boot.

Yes sound like brake booster. most of time when booster is going bad makes hissing sound when brake applied.But should not make any kind of air leakage noise at all.Replace brake booster.Hope this was helpful for you.

you mean you hear air in the cab down by the pedal. If thats the case your power steering assist is leaking. Its the big black unit behind the brake master cylinder. It may set a check engine light at times for lean condition.

brake booster may be bad,

If the brake pedal makes a crunching noise,try flushing out the brake fluid. For some reason after flushing the noise goes away. I hope this helps,goodluck.

Sounds like your brake booster but can't be sure until I know what kind of truck or car you have....

If it is a constant hiss with your foot on the pedal, you have a bad brake booster. If you hear a hiss, and nothing else while holding the pedal, it is the air escaping from the booster boot, and should not pose a problem.