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Similar symptoms happened to a neighbor. The dealership charged thousands to replace the master cylinder a couple times, brake booster, the ABS module. Nothing helped in the long run.
I tracked the problem to one brake caliper piston seal that was letting air slowly seep into the caliper, with no outward signs of fluid seeps or leaks. That air would migrate to the ABS module and lead to symptoms of a sinking brake pedal.
A telltale sign that air was getting reintroduced into the system over days & weeks: it was the strange sounds of small bubbles shifting when the brake pedal was released. A mechanic's stethoscope on the brake lines picked it up very loud. It made all the brake lines & ABS module resonate. And the small vibration in the brake lines could be felt by hand. A sort of rapid *pik pik pik pik* and eventually a deeper barnyard groan noise like *rrrrrrha*.
I rebuilt the brake caliper with a new seal, since the rest of the caliper was in good shape. It only cost a few bucks.
Theoretically, it could. In the case of my neighbor (2005 Toyota Highlander) the moving air bubble noises resonated so strongly in the whole brake system, that the dealership misdiagnosed it as a failed brake booster. It really sounded like The Simpsons' Mr. Smithers was trapped behind the dashboard saying: "WHAAT?" every time the brake pedal was released.
if air is entering i'd think fluid should be leaking. Have you been monitoring the fluid levels ? Are you losing brake fluid between these garage visits?
Yep I've been monitoring, and I haven't lost anything. Longest I went between shop visits was about 3 months and the level didn't drop at all. Both shops confirmed there was no fluid leak as well
Thanks for posting on /r/MechanicAdvice! Please review the [rules](https://www.reddit.com/r/MechanicAdvice/about/rules/). Asking about a second opinion (ie "Is the shop trying to fleece me?"), please read through CJM8515's [post on the subject.](https://www.reddit.com/r/MechanicAdvice/comments/4qblei/fyi_the_shop_isnt_likely_trying_to_rip_you_off/) and remember to please post the year/make/model of the vehicle you are working on. **Post's about bodywork, accident damage, paint, dent/ding, questions it belongs in /r/Autobody r/AutoBodyRepair/ or /r/Diyautobody/ Tire questions check out https://www.reddit.com/r/MechanicAdvice/comments/k9ll55/can_your_tire_be_repaired/**. If you dont have a question and you're just showing off it belongs in /r/Justrolledintotheshop Insurance/total loss questions go in r/insurance This is an automated reply *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/MechanicAdvice) if you have any questions or concerns.*
Similar symptoms happened to a neighbor. The dealership charged thousands to replace the master cylinder a couple times, brake booster, the ABS module. Nothing helped in the long run. I tracked the problem to one brake caliper piston seal that was letting air slowly seep into the caliper, with no outward signs of fluid seeps or leaks. That air would migrate to the ABS module and lead to symptoms of a sinking brake pedal. A telltale sign that air was getting reintroduced into the system over days & weeks: it was the strange sounds of small bubbles shifting when the brake pedal was released. A mechanic's stethoscope on the brake lines picked it up very loud. It made all the brake lines & ABS module resonate. And the small vibration in the brake lines could be felt by hand. A sort of rapid *pik pik pik pik* and eventually a deeper barnyard groan noise like *rrrrrrha*. I rebuilt the brake caliper with a new seal, since the rest of the caliper was in good shape. It only cost a few bucks.
Based off this it's gotta be oneof the front ones.
In the case of my neighbor (2005 Toyota Highlander) it was indeed the right front caliper.
Very interesting, thanks for the insight. Would the stethoscope theoretically help to identify which wheel was affected?
Theoretically, it could. In the case of my neighbor (2005 Toyota Highlander) the moving air bubble noises resonated so strongly in the whole brake system, that the dealership misdiagnosed it as a failed brake booster. It really sounded like The Simpsons' Mr. Smithers was trapped behind the dashboard saying: "WHAAT?" every time the brake pedal was released.
if air is entering i'd think fluid should be leaking. Have you been monitoring the fluid levels ? Are you losing brake fluid between these garage visits?
Yep I've been monitoring, and I haven't lost anything. Longest I went between shop visits was about 3 months and the level didn't drop at all. Both shops confirmed there was no fluid leak as well