HISSING SOUND COMING FROM UNDER MY DASH NEAR BRAKE PEDAL????

[Askernie]

I have a 1990 Mustang GT convert. I noticed about a week ago a hissing sound coming from under the drivers side dash area. When I press the brakes the hissing sound changes from a high sounding hiss to a lower pitch hissing. It hisses all the time, idle and driving.

What is this? I looked under the dash and didnt see any vacuum hoses that could be leaking.

Im completely stumped???

Ernie the Eyeball

 

[kendoo130]

Theres a vacuum line hooked up to the brake pedal to de activate the cruise control-look under there really good and you'll see where the hose goes.

 

[SeussStang]

You might also check your power booster, you may be getting a leak from that, I wouldn't think you would hear that inside the car so much though.

 

[wythors]

Theres a vacuum line hooked up to the brake pedal to de activate the cruise control-look under there really good and you'll see where the hose goes.

Say WHAT?

You brake booster is leaking through the firewall. Time for a new booster.

 

[Maryland Stang]

Yup! Power brake booster for sure. The sound should change when you hit the brakes.

 

[MustangLX-5.0]

It's the brake booster pushrod seal.Or a seal on a small vaccum port inside.Same thing happened with mine.Only thing was,it hissed a lot when I pushed the pedal in about half way.

There is no vaccum line for the cruise control.The cruise deactivate is controlled by a BOO switch (Brake On/Off) near the top of the pedal.