hissing when hitting the brake pedal - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jklonowski · 10-17-2010, 12:08 AM

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.

hugger6933 · 10-17-2010, 08:04 PM

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

justinburnett · 10-17-2010, 11:07 PM

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

f.monroe · 10-17-2010, 11:16 PM

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

motornut · 10-18-2010, 09:46 AM

if not the booster,look here

boatpuller · 10-18-2010, 10:25 AM

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.

jklonowski · 10-18-2010, 07:22 PM

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

10-17-2010, 12:08 AM	#1
jklonowski Registered User Join Date: Feb 2009 Location: franklin kentucky Posts: 70	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. Last edited by jklonowski; 10-17-2010 at 12:09 AM.

10-17-2010, 08:04 PM	#2
hugger6933 Registered User Join Date: Oct 2008 Location: Marianna Arkansas Posts: 7,262	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

10-17-2010, 11:07 PM	#3
justinburnett Registered User Join Date: Sep 2010 Location: The Dalles, OR Posts: 392	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.

10-17-2010, 11:16 PM	#4
f.monroe Senior Member Join Date: May 2004 Location: Caldwell,Tx. Posts: 3,648	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 __________________ Born and proudly residing in a Red State !

10-18-2010, 09:46 AM	#5
motornut 78K & 79C Jimmys Join Date: Nov 2009 Location: Ottawa Ont CANADA Posts: 7,901	Re: hissing when hitting the brake pedal if not the booster,look here Attached Images __________________ John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350 Last edited by motornut; 10-18-2010 at 09:47 AM.

10-18-2010, 10:25 AM	#6
boatpuller Registered User Join Date: Sep 2005 Location: Sun City West, Az Posts: 3,822	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. __________________ " Never look up a dead dogs butt."

10-18-2010, 07:22 PM	#7
jklonowski Registered User Join Date: Feb 2009 Location: franklin kentucky Posts: 70	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 Last edited by jklonowski; 10-18-2010 at 07:24 PM.



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