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Discussion Starter #1
Been noticing a hissing sound when I apply the brakes. The sound is right under the dash near the brake pedal, and seems to go away if I press on the brake pedal pretty hard. If I just lightly press it, then I hear a hiss.

What could it be? (Or is it normal?) I only recently noticed it, but that doesn't mean it hasn't been happening for a while.

Thanks.
 

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That would be the vacuum release for your cruise control (kicks it off when you apply the brake). It may just have slipped out of adjustment. Good luck.
 

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MY Dodge did the very same thing, only under light braking, full-on braking and it was OK (just like you're describing) and it turned out to be a major vacuum leak at the power brake booster. New brake booster and it was fine. Car ran a lot better also without the vacuum leak.

Good luck...
 

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Definately check at the brake booster for vacuum leaks. It sounds like you have a cracked line near it, or the booster seal itself is torn/cracked causing the hiss.
 

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Discussion Starter #5
I basically noticed this after I took my upper intake off to change out the fuel injector o-rings. After I put the intake back on, and started up the car (and I made sure I hooked up all the vacuum lines I disconnected for the intake removal) I start hearing this hideous hiss when I apply brake.

So, last night I pulled the intake again (this time to replace the injectors - the o-rings didnt' work. :( ) and put it all back together and notice the hiss. (I haven't driven the car in 3 weeks until today)

It really sounds like the hiss is coming from under the dash (i.e., not in the engine bay) and I looked under the dash today and pressed the brake w/ my hand. The little bellows and the rod (I guess it goes to the brake booster?) .. it seems like it's hissing right there.

Where would I look for vacuum lines, and is there any way I can check the booster, or should I just have a mechanic look at it? I don't know squat about brakes/power boosters/etc,etc.

Thanks for the help, btw.
I couldn't do this hobby w/out you guys/gals.
 

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That would be the vacuum release for your cruise control (kicks it off when you apply the brake). It may just have slipped out of adjustment. Good luck.
Yes it is this...... I had the same problem once.. its located on the brake pedal, just follow the pedal up to the dash and you will see the vacuum line and vacuum switch, its black with a white nub coming out of it which is depressed by the brake pedal. Good luck, just climb under there with a flashlight and look for it, trust me its easy.
Lugnut
 

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"It really sounds like the hiss is coming from under the dash (i.e., not in the engine bay) and I looked under the dash today and pressed the brake w/ my hand. The little bellows and the rod (I guess it goes to the brake booster?) .. it seems like it's hissing right there. "


Sounds like the power brake booster to me, what that has to do with the intake install...I dunno, maybe coincidence. Stick your hand up there and see if you can feel it at the plunger when depressing the brake pedal. I would think if it was the cruise control vacuum solenoid it would hiss all the time if the hose was loose or disconnected, doesn't the loss of vacuum at that diaphram trigger a sensor to reset the cruise control? In other words doesn't the plunger depress(or release) when the brake pedal is depressed interuppting the vacuum source?
 

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Lots of 5.0 experience said:
That would be the vacuum release for your cruise control (kicks it off when you apply the brake). It may just have slipped out of adjustment. Good luck.
I thought exactly the same thing. My mom's old Marquis had that problem, so I changed the vacuum switch and took car of it. But did Mustangs ever come with this option???
 

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Discussion Starter #10
One more thing.. it seems like the pedal has to be pushed pretty hard to get the car to stop well. Almost to the point of locking up, but not quite.

I'm going to look for that vacuum line probably tonight. If that checks out ok then I guess it's a trip to the mechanic. :(
At least I can trust this mechanic... just dollars down the drain though.
 

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my car had the same problem...turned out to be the brake booster
 
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