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Sinking brake pedal mystery

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  #1  
Old 09-14-2018, 07:16 AM
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Default Sinking brake pedal mystery

My new to me 2002 has been hanging around various shops for days with a sinking brake pedal. The brakes work fine but the pedal goes to the floor, or close to the floor, during application.

The system has been bled multiple times, the last time was with a power bleeder and a number of new master cylinders have been installed. The calipers appear to be functioning properly and the computer shows no errors in the system.

In looking through this and other forums, the smoking gun always seems to be air in the system, a bad MC or mis-installed clips on the front caliper.

Are there any other things that might cause this behavior? Could the ABS be involved?
 
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Old 09-14-2018, 07:19 AM
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Sounds like a brake booster has failed.
 
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Old 09-14-2018, 07:39 AM
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that was my thought initally becase the actual brakes are working fine, but the mechanic felt that a failed booster would lead to a hard pedal, not a sinking pedal?
 
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Old 09-14-2018, 11:34 AM
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There are only two things ahead of the brake pedal, the booster and the master cylinder. If you are satisfied that the master cylinder is holding pressure, then it has to be the booster.

One way to diagnose this issue is to hold the brake pedal and let it hit the floor or maximum travel. Then, while holding the pedal down, have someone open a bleeder screw. If you get very little fluid, then it is somewhat safe to say that the master cylinder is bypassing. If you get a large quantity of fluid, then the master cylinder is holding pressure.

I've not had a Volvo brake booster apart.
 
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Old 09-14-2018, 11:54 AM
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thanks for that trick. Can I bypass the booster for testing by removing vac hose? I'd hate to replace it just for fun.
 
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Old 09-14-2018, 03:20 PM
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No. There is still a mechanical connection inside the booster.
 
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Old 09-14-2018, 03:32 PM
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Originally Posted by tony1963 View Post
No. There is still a mechanical connection inside the booster.
so if that's the case, would a failed booster result in a sinking pedal?
 
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Old 09-14-2018, 05:19 PM
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any wheezing or hissing sounds when you pump the brakes?
 
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Old 09-28-2018, 05:40 PM
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It's been a while but the problem is finally solved. A third mechanic replaced the ABS motor, power bled the brakes twice, isolated individual calipers, etc. etc.

The fix?

A 5 dollar check valve on the booster

Thankfully, the seller dealt with all this nonsense. Now I own this high maintenance trophy wife of a car, hopefully it won't break my heart.
 
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