Need help - brake pedal losing pressure

[GJarrett]

When I hold my brake pedal down with pressure (like at a red light) it will slowly and gradually move down to the floor. If I pump it quickly and hard I can hear air hissing during each pump. There is no brake fluid leakage anywhere. I am assuming there is some sort of air piston that transfers brake pedal pressure to the master cylinder (?). Can a bad master cylinder act this way?

Can someone enlighten me on what is happening, what the fix is, how easy to DIY, etc?

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[SHOdaddy68]

Sounds like a leak somewhere. A caliper, line or the booster. If you're not loosing any fluid and the reservoir level is good, my guess is something in the booster. Replacing a booster/master is not a fun or easy task. If you don't have the expertise to diagnose and fix it yourself, don't compromise your safety. Take it to a shop and have it checked out ASAP.

 

[GJarrett]

Yeah I'm not losing fluid... I'm a pretty competent DIY but not exprienced with boosters and cylinders, and too old to be in the mood to mess with a job like that, if that's what it is.

If it is a leaky booster will it keep gradually getting worse or can it decide to catastrophically completely fail instantly?

 

[SHOdaddy68]

It will probably continue to degrade. Don't know if it will or could completely fail but I would not take a chance with it.

 

[Unas2k5]

Brake booster definitely since there’s hissing

 

[GJarrett]

Thanks for the insight. I have a trusted local shop and will take it there to get confirmation and estimate.

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