Brake pedal hiss - time to replace?

[Papagut]

Hi there,

I started getting a little squeek at the top end of the movement as the brake pedal was released, so I have been paying closer attention and now hear a slight hiss. My understanding is that this is the vacuum from the brake booster escaping, and that indicates that is time to replace the part. Otherwise, the brakes feel fine, and no fluid is leaking from the system. So, I have a few questions before I undertake this:

What is the correct term for the part that is failing, I feel like a dope not being able to refer to it properly.

Is this a serviceable part? Can I replace a seal to correct the leak?

What causes this failure? The brake fluid has not been changed in a while, will fresh fluid re-lubricate the system and correct the leak?

Finally, a part number would be helpful or any other recommendations on the repair before I get going on it.

Thanks guys.

[wvicary]

You can often here a hiss noise when booster is working correctly. How do the brakes feel? If there was a problem you should have warning on the display in short order.

[Papagut]

To me they feel fine, but the squeek was the first symptom, then a hiss as the pedal came back up to the ready position that I feel is becoming more pronounced. I have seen a brake booster warning once now too on the dash - came up once, not seen again.

[wvicary]

Inspect the system for vacuum leaks often caused by a split hose. The squeak could be just a dry pivot point.

[Bansasn]

Here is a link what was talked about on "Brake Booster Faulty"http://www.6speedonline.com/forums/cayenne-955-957/281093-brake-booster-warning-troubleshooting.html . I fixed my vacuum lines with high temp. electrical tape and metal clamps. Saved me $600 bucks!

[ciaka]

If your brake booster itself was bad, you would know right away because you would have to stand on your brakes to stop the car. without brake booster you have to apply extreme forces to brake pedal, to make a car stop. If the brakes 'feel' fine, it might be something else, like vacuum lines, others spoke about. If you are trying to be proactive, it may be premature to replace (unless you have money and time to change, and even then you may be replacing a well functioning part).

[Papagut]

Thanks - I will look into it a little more closely and post back the results. I was imagining the seal between a piston and chamber beginning to fail, and the wearing out of the seal allowing the air to leak into the vacuum system. I will post back with the results.

[9552T500]

Hi - had the brake booster failure coming up on the cluster like out of the blue... Heard noisy vac pump and realized that vacuum line next to the RHS airbox was loose due to crack . Here some pix of the problem and temporay (wife is scared of dashboard alerts and just re-setting before the spare comes isn't nice ....) fix to share: 06_CayenneTurboS_troubleshooting_brake_booster_failure_fix . Hope this helps somebody. Good luck & Enjoy fixing the turbo pig !

[hahnmgh63]

Great pics, something for everyone to check.

[Papagut]

It's been a while, but I have an update on the squeek / hiss I was getting as the brake pedal is released.

I did check the vacuum line and booster pump, that does seem relatively common for causing some issues. I had thought that perhaps a split and weak vacuum was causing what I was hearing in the cabin - but no, everything was in good shape.

So, I removed the panel in the cabin where the pedals are and with the engine running - and it took a while to locate - I identified the culprit as the rubber boot that surrounds the rod that the brake pedal depresses. To elaborate: the boot is fixed to the end of the rod that is closest to the pedal, and when you push it in, it collapses the boot like an accordion - as it should. However, when I released the pedal, the other end of the boot pulled away from the back wall also, and as it did, a hiss and squeek would be produced.

It is hard to access, but if I worked my hand to the back of the boot and just squeezing or moving it a little would produce the same hiss. It didn't make sense for the boot to move at that back end, so I pushed the base backward. I believe that the back end of the boot had come loose, and sliding it backward remounted it - although it probably only moved back maybe 5mm.

Once done, when depressing the pedal, the back of the boot is now stationary, and it collapses / expands as it should - more importantly, it instantly eliminated the squeek and hiss.

I'm not certain how long this fix will last, or if the boot coming free is a sign of something else (my brake booster pump was getting fairly hot - weak vacuum, but not enough to trigger fault?), but for now, the problem I originally posted is resolved.

Picture of the boot attached - hope this helps if you are experiencing anything similar.

Mark

Edited April 28, 2013 by Papagut