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Brake pedal randomly gets soft

Tiny
WALLY AB
  • MEMBER
  • 2008 TOYOTA YARIS
  • 1.5L
  • 4 CYL
  • 4WD
  • AUTOMATIC
  • 1,250,000 MILES
Had this car more than three years. Car has no ABS module so that is out of the question. Pedal always felt somewhat soft. Replaced a few master cylinders thinking it was the problem, and forgot about it since that was just the way this car was and it stopped. Last time I had a brand new original unit and a few weeks later I started getting random soft pedals at times, rarely though. I returned the unit, and put back my original on, still same thing. I bled the lines from the front two lines connecting to the master cylinder and never had a problem with air in the system. My rear drums, shoes, wheel cylinders were all replaced in the spring, Front brakes are still fine, plenty of meat on them, but slight warpage on the rotors. There are no leaks in the lines or anything. I am stumped as to what it may be. Can it be air still stuck somewhere in the system, or a bad booster? I remember having a rear seal leak with one of the old master cylinders and cleaning the brake fluid out of the booster. Can this also cause the symptom?
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Thursday, December 28th, 2017 AT 6:41 PM

17 Replies

Tiny
KHLOW2008
  • MECHANIC
  • 41,814 POSTS
Hi Wally,

It could be due to some air still trapped in the system, a fault with the brake proportioning control valve (PCV) or the master cylinder is not in perfect condition.

With engine running and vehicle stopped, depress the brake pedal and hold pressure and position. If the pedal sinks after a few seconds, that would mean the master cylinder seals are leaking internally.

If above are good, then the brake PCV would be the likely cause. It is intermittently loosing residual prressure and when that occurs, pedal travel would have to be longer to get full hydraulic pressure in system.
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Sunday, December 31st, 2017 AT 12:33 AM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
I bled the brakes at each wheel and found that at the rear there was some air. I thought air in the system would be noticable right away all the time
The master cylinder doesn't sink so I know it's fine. The Yaris is also known for not having the best brake feel as well
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Sunday, December 31st, 2017 AT 6:27 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
The brake rotor warpage will cause a low pedal because it backs the pads up. Also I would check the wheel bearing is see if you have one going out allowing the rotor to wobble which will also back the pads up.
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Tuesday, January 2nd, 2018 AT 11:33 AM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
The front brakes have about 40% left on them. The warpage is minor. Many times you dont feel it. I did bleed the lines again front to rear last week and found the back lines had some air in the system. Also found the passenger right rear wheel cylinder bleeder had dirtier fluid even though it was changed not too long ago.
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Tuesday, January 2nd, 2018 AT 5:49 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
This sounds like you may have a seized caliper slide which will back the pads up as well. Here is a guide to help you see what I am talking about.

https://www.2carpros.com/articles/how-to-replace-front-brake-pads-and-rotors-fwd

With air in the rear the rear wheel cylinders could be sucking air in as well.

https://www.2carpros.com/articles/how-to-replace-a-wheel-cylinder

Please let us know what happens.

Cheers, Ken
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Wednesday, January 3rd, 2018 AT 5:41 PM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
The front caliper slides were greased a few months ago. I know this many not make a difference but the brand I used on the rear wheel cylinders we're slightly smaller and the adjuster had to be turned to compensation. I used centric premium wheel cylinders
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Wednesday, January 3rd, 2018 AT 7:33 PM
Tiny
KEN L
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  • 47,513 POSTS
That shouldn't make a difference. Here is something we can do to isolate the problem. Can you get 3 vise grips and gently close off the rubber brake hose by clamping down on them use a piece of cardboard to protect the hose from damage and they don't need to be super tight just enough to block the hose. Once the vise grips have been installed see if the pedal fades if so the master is to blame if not then the problem is out at one of the wheels.

Please let us know what you find. We are interested to see what it is.

Cheers, Ken
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Thursday, January 4th, 2018 AT 11:11 AM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
Didn't notice any master cylinder fade. This happens rarely. It might just be the slight warpage or misalignment. I don't think I've even noticed it since I bled the brakes last, except on snow which could be caused by skidding
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Thursday, January 4th, 2018 AT 11:36 AM
Tiny
KEN L
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It could be because of the loose wheel bearing as well, can you raise the front of the car and grasp each front wheel to see if they wobble?

https://www.2carpros.com/articles/jack-up-and-lift-your-car-safely

and

https://www.2carpros.com/articles/how-to-replace-front-wheel-bearings-and-seals

Please let us know what you find.

Cheers, Ken
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Friday, January 5th, 2018 AT 11:02 AM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
I checked the wheels for play and there is none, nor a sound coming from the wheel bearing
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Saturday, January 6th, 2018 AT 8:22 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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It sounds like the master cylinder must be bad then its the only thing that can cause to pedal to fade.

Here is a guide that can help

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

and

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Please let us know what happens.

Cheers, Ken

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Saturday, January 6th, 2018 AT 12:17 PM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
Today I pressed on the brakes felt engage pulsating, then it became soft, pressed on it again and it was fine. Only happened once. I really don't suspect the master cylinder since I replaced it and had same symptom. Maybe front pads, rotors and good lubrication?

I just greased the slides and brake pad ears
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Saturday, January 6th, 2018 AT 1:46 PM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
I meant to stay I greased it right after I felt the pedal felt low and pulsating. Only real thing I can point to is front brakes
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Saturday, January 6th, 2018 AT 3:49 PM
Tiny
KEN L
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Warped rotors can cause an intermittent low pedal.
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Sunday, January 7th, 2018 AT 10:22 AM
Tiny
WALLY AB
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  • 97 POSTS
Can a bad check valve cause a low pedal? Sometimes I feel like I have to press twice on the pedal to get it to engage, but I dont feel it sinking.
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Friday, January 12th, 2018 AT 8:20 PM
Tiny
KEN L
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You can check the check valve by starting the engine and then shutting it off. Then remove the check valve from the booster and you should hear vacuum being released.
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Saturday, January 13th, 2018 AT 6:45 PM
Tiny
WALLY AB
  • MEMBER
  • 97 POSTS
I mean booster.
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Saturday, January 13th, 2018 AT 6:55 PM

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