Hissing sounds in cab when on brakes

[Hugemoose]

I know that sounds strange, but I don't know how else to describe it. Every time I hit the brakes I hear a hissing sound....simply moving air...inside the cab. It sounds like it is coming through the drivers side firewall....near the brake pedal! To get an idea of what it sounds like leave your mouth slightly open and simply blow. The harder I hit the brake the louder the sound gets. As I back off of the brake when coming to a stop it gets quieter but is still present while the pedal is depressed.

Also, when I hit the brake pedal very quickly while moving, like in an emergency type situation, the pedal depresses but the brakes feel like they don't apply for almost an entire second or two. It's seriously nerve-racking!! To be honest I am more worried about this!!!

I am thinking that maybe my booster might be taking a dump?! I really have no idea. Please help

[MEvang]

Power boosters work by using vacuum on a large diaphragm to assist in the braking action. Vacuum can only apply the brakes when atmospheric pressure is available on the opposite side of the diaphragm.When you are not using your brakes the atmospheric pressure is sealed off and the vacuum cannot apply your brakes on it's own. When you hit the brakes, atmospheric pressure rushes into the booster and then the vacuum can assists in stopping. When this happens a whoosh of air can sometimes be heard, as the atmospheric pressure enters the chamber. This is muffled by foam around the booster brake push rod that attaches to the pedal.
So, even if this foam has deteriorated as it sometimes does, you should only hear the first rush of air, after that nothing, If you continue to here a hiss coming from the booster while your foot is on the pedal, you have an internal vacuum leak. This will cause the booster to be less efficient. I would say you need to replace the booster.
Mike

[Hugemoose]

Alright thank you. You really went into great detail with that too......I appreciate it. I had a feeling that this might be the case. I will look into it and see how hard it will be to replace. God knows I don't want to take it to a dealership......

And I just bought new leaf springs

[MEvang]

Most boosters are fairly easy to replace. You don't even need to take the lines off the master cylinder, just swing it out of the way. Getting to bolts under the dash is usually the hardest part.
Mike

[Hugemoose]

Thanks Mike. I am having my friend price one out for me today at the dealership where he works.

Thanks again for the diagnosis help. It would probably have ended up at the dealership if I couldn't figure it out....and I didn't want to spend that kind of money.

[Hugemoose]

On Thursday I picked up a brake booster for $264 plus a $40 core charge. And this was with a discount I get from my friends at Advanced Auto....it's basically a corporate customer discount. Regular price was $323.

The dealers price (at dealers cost) was $435. Ouch!

It should be in the end of the week and hopefully I will have it in my truck this upcoming weekend. Thanks again for the help Mike.

[Hugemoose]

Update:

I got the brake booster in this evening and everything is working great! The pedal feels great and everything seems to be working perfectly. After everything was done we bled my brakes which was needed, and nice because my old fluid was a little dirty.

But, it was no cake walk to get out. The MC did have to be completely removed and I had to unbolt brake line brackets and the carbon canister in order to make room for the booster to come out. The bolts on the inside weren't too bad to reach but I did have to remove the lower interior panel below the steering wheel and the air duct under that. Both were fairly easy to remove and provided enough room to work down there with relative ease.

Now on Friday I'll take my old booster back and get my $40 back!!

Thanks again for the information Mike. This was so much better than going to a dealership