Brake pedal hisses while depressed

Answer

It's the hydraulics in your pedal, and it's normal to hear some slight hissing. If you hear a lot of air, and it seems like your breaks aren't working, then have it checked out.

If the hissing is louder than normal or gets louder as you depress the brake, it could very well be the power brake booster. If it almost stalls the engine while holding the brake down, this would indicate an excess loss of vacuum in the system, which helps control the engine operation. If this is the case, there is a good chance the booster has gone bad, and needs to be replaced.

One other item to look for is less responsive braking than normal, in other Words, you are having to push down further or harder than normal to slow or stop the vehicle. The power booster is what gives you "power assisted brakes".