Brake Pedal is Soft and Hissing

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Jan 15, 2019 | 06:01 AM

markd248

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1st Gear

Joined: Mar 2016

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Brake Pedal is Soft and Hissing

Hey Everyone, please help me diagnose this problem -

I started my car Saturday after not driving it since Wednesday and my brakes were extremely soft and hissing whenever I stepped on the pedal. The pedal almost feels like it is not receiving any pressure until it is about halfway pressed down and the brakes bites very hard. While I'm depressing the brakes the pedal hisses. It sounds like it is coming from inside the cabin above the brake pedal.

My immediate thought was that it was a diaphragm leaking on the brake booster, but it sounds like that'd result in an extremely stiff brake pedal. From the research I've done, it sounds like it might be the Power Brake Vacuum Line B. Which is this line.

My car also has the p2096 code, but that's shown up off and on for a year, so I don't think its connected to (potentially) the vacuum line.

Could anyone give me advice on troubleshooting this problem or how hard it'd be to replace this vacuum line?

Thanks!

Jan 15, 2019 | 06:09 AM

Tgriffithjr

5th Gear

Joined: Jul 2018

Posts: 766

Likes: 80

From: Texas

If it’s hissing when you mash on it then it’s a vacuum leak in the booster. It is leaking until the applied pressure overcomes the leak and then it suddenly applies the brakes. You can have a leak and still have vacuum assist. I’ve seen this in my own experience. Try disconnecting the vacuum source and then attempting to apply the brakes. Be sure to plug the vacuum line so you don’t lose vacuum. Do this in a safe place such as your driveway. If you are hearing the hiss inside the car then it’s coming from the booster. You wouldn’t hear the line leaking inside the vehicle. Have someone apply the brakes while you listen under the hood. You don’t need the engine running for this test as the system should hold vacuum.

Jan 15, 2019 | 11:29 AM

markd248

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Joined: Mar 2016

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So, using the image above, I should plug the vacuum line and the booster to separate the two for diagnosing where the leak is coming from and then step on the brakes? It sounds like your leaning towards it being the booster since the leak can be heard inside the car?

Thanks for your help so far.

Jan 15, 2019 | 03:44 PM

Tgriffithjr

5th Gear

Joined: Jul 2018

Posts: 766

Likes: 80

From: Texas

Don’t bother unplugging anything. Just run the engine to make sure you build up enough vacuum. Shut off the engine and then mash on the brakes. Have someone listen under the hood while you listen inside to determine where the hiss is coming from. If you hear it inside then it’s the booster that’s leaking.

Jan 16, 2019 | 05:51 AM

Tgriffithjr

5th Gear

Joined: Jul 2018

Posts: 766

Likes: 80

From: Texas

Here is a link that explains how power brakes work. https://auto.howstuffworks.com/auto-...wer-brake1.htm

Jan 16, 2019 | 06:03 AM

Tgriffithjr

5th Gear

Joined: Jul 2018

Posts: 766

Likes: 80

From: Texas

Here is a link that will help you diagnose the problem. It’s difficult to diagnose a problem correctly from long distance. http://www.agcoauto.com/content/news/p2_articleid/129