Hiss when I step on the brake

Where does the noise sound like it is coming from? Could it be coming from outside the vehicle?

My son stepped on the pedal when I listened under the hood.

Under the hood I hear nothing - not from hoses nor the front of the booster. I unplugged the rad fan so that I could hear better.

I > Where does the noise sound like it is coming from?

I think you're describing the sound that every car makes when you step on the brake.

If it's a steady hiss that lasts after the pedal stops moving down then he's probably right about it being the booster. Try pinching the booster vacuum hose with vice grips and see if it goes away.

Tim

I have never heard a continuous hiss. Not continuous for as long as I press on the brake pedal with the engine running. It does not matter if I hold with ateady pressure or if i pump the pedal - the hiss is there - if i press and hold for 5 minutes the hiss is steady for 5 minutes. I don't think this is normal.

When I shut off the engine and then pumped the pedal repeatedly to remove the vacuum and then pressed the pedal to it's maximum travel it stayed in that position - but was not bottomed out. Then when I started the engine the pedal "fell" a bit more. This position had no hiss because it was beyond it's half-way travel position.

I think I'll talk with a journeyman mechanic friend for his opinion....

....and then some vehicles have a vacuum hose going to the brake light switch under the dash in order to cut off the cruise control when brakes are applied. Said switch might just leak/hiss when pedal is partially applied and then quit leaking/hissing once the vacuum-release plunger has been depressed beyond its "critical initial" depression! Just a thought. Good luck, sdlomi2.

The booster has a leaky valve or a tear beginning in the diaphragm. It will probably fail eventually, and replacing it is about the only option. Since the hiss stops when you press harder on the brake, my guess is that it is NOT the diaphragm with a tear, but the valving that allows atmospheric pressure into one half of the booster to press against the diaprhagm. It is supposed to cut off vacuum from that side of the diaphragm at the same time it allows atmospheric pressure in, but sometimes they get a bit sticky. It could continue to work that way, just fine, for 20 years. Keep an eye (ear?) on it, and if it seems to get worse then replace the booster.

This valve issue is what a former student of mine (now a journeyman - trained on Chrysler units - and an Auto Tech teacher) told me. So it means no basic issue right now, but it could die. Then replace. By the way, this booster is possibly 20 years old since the van is a 1984 model. Still works fine, but still needs work - constantly ;-)

Thank you EVERYONE for ideas etc - and for several private e-mails!

Ken

This reminds me that some vehicles have an auto release for the parking brake. Does this hiss occur even if the vehicle is in park?