hissing when hitting the brake pedal
 

my truck is making a hissing noise when i hit the brake pedal the idle raises which clearly is a vaccum leak. well i was told it was the vaccum booster, well i replaced it today and it didn't take care of the hissing when i hit the brakes. i don't know what it is so a buddy and i are going to tear into the dash. it sounds like it's coming from behind the ac controls above the radio but i'm not sure. i can't hear any kind of hissing under the hood but i also have headers to cherry bombs to 3 1/2" tips in front of the rear tires so it's kinda hard to hear anything. any ideas where the leak could be coming from would be greatly appreciated.

Re: hissing when hitting the brake pedal
 

The flapper doors that control the movement of the heat and air system are vaccum controlled. The movements of the flow are also[ from top of the dash to center to floor]. Your vac leak is probablly right where you thought it was above the radio. Jim

Re: hissing when hitting the brake pedal
 

I had a jeep that was the same way. It was a vac actuator behind the dash for the heat/def/ac selector.

Re: hissing when hitting the brake pedal
 

Was the new booster used or new? If it is used then it is also bad. The only thing that would cause a hiss when you press the pedal would seem to be a bad booster. If you want to be sure , disconnect the vacuum from the booster and cycle the pedal a couple times and see if the noise is gone.
Frank

Re: hissing when hitting the brake pedal
 

if not the booster,look here

Re: hissing when hitting the brake pedal
 

iv'e had that happen on a couple of rigs in the past and both times like frank said,it was the booster.but.since you replaced it.it seems kinda odd it would be bad.

Re: hissing when hitting the brake pedal
 

solved it after i changed the booster and went ahead and changed the master cylinder as well. it was a vacuum plunger like switch that was connected to the brake pedal. i guess it was used to kill the cruise when the brakes we engaged. it used a vacuum line that was "T" from the brake booster line and went through the firewall down where that ball is next to the brake booster. sucks i wasted over a hundred dollars just to disconnect a vacuum line. awe well at least i don't have to worry bout them for awhile i hope lol