Hissing Coming from brake pedal??

[Mischi]

The other day I noticed a faint hiss coming from under the drivers side dash. When I pushed the brake pedal the hiss went away. Yesterday morning the hiss was much louder, I'm assuming it was due to colder temperatures. When the car warms up the noise is gone. Again when i press the brake pedal the noise goes away. Any ideas?

[Towncivilian]

Your brake booster check valve may not be sealing properly any more. After 12 years, this isn't so farfetched. Open the hood and have a helper press on the brake pedal with the engine off with everything cold, and listen around the brake booster for a hiss. If you can't hear it, try it with the engine on. It could also be a vacuum leak, but in this case you would have an unstable idle.

[iFly QX4]

I just started having the opposite issue from Mischi. When I depress the brake pedal I have a hissing sound coming from the brake booster. I discovered my vacuum line from the booster to the check valve was cracked, so I replaced it with a non-premolded hose from Autozone. The check valve seems to be operating.

[farfrom]

I AM HAVING A SIMILAR ISSUE (THE FIRST TIME) WITH MY '96 PATH...

WHEN I INITIALLY ENGAGE THE BRAKE PEDAL (PRE-START) I HEAR A FAINT, BUT NOTICEABLE, HISSING--AS IF DEFLATING A BALL/TIRE--COMING FROM THE BOOSTER AREA.

I HAVE NO DISCERNIBLE PROBLEM WITH THE PERFORMANCE OF THE BRAKES, AS THEY FEEL, AND OPERATE (SEEMINGLY) AS THEY NORMALLY WOULD.-- IF IT WEREN'T FOR THE DEFLATION SOUND, I WOULD NOT SUSPECT ANY PROBLEM WITH THE SYSTEM.

COULD THIS BE A SYSTEM LEAK, AS WITH iFLY'S QX4 CRACKED LINE? OR, SOMETHING MORE SINISTER?


THANKS!