Some drivers report a soft brake pedal and suspect the cause is air in their ABS (after they've done a standard bleed on all calipers). For most ABS systems, I understand the ABS remains in a passive/off mode during normal driving and only activates when it senses an extreme imbalance in wheel speed during braking. In this case, if the ABS is a closed circuit during normal driving, how can air that somehow got into the ABS have any effect on normal braking and cause a soft brake pedal?
Air in ABS Brake System - impact on normal braking?
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the ABS pump is also part of the normal brakes. The electronics of the ABS don't normally need to intervene until it is necessary
Air shouldn't be getting into the ABS module or the brake system downstream of said module unless the master somehow goes dry.
And yes, it will impact brake system function. I had a ticket last year on a Kia Sportage that had been driven with a dry master for who knows how long, and it drove me batty for a few days. Wasn't until we did an ABS bleed after replacing the master that we had a pedal again.
And yes, it will impact brake system function. I had a ticket last year on a Kia Sportage that had been driven with a dry master for who knows how long, and it drove me batty for a few days. Wasn't until we did an ABS bleed after replacing the master that we had a pedal again.
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Until a few year ago, I would agree that air (gas) could only get into the hydraulic system if the master cylinder reservoir had run dry, or had a leak in the system. A central heating engineer suggested this: That gas can be generated from the electro-chemical reactions of the various materials in a brake's hydraulic system, including corrosion, and are accelerated by the brake system's heat cycling. How otherwise can brake pedals sometimes go soft during use?
Originally Posted by Donald64
Until a few year ago, I would agree that air (gas) could only get into the hydraulic system if the master cylinder reservoir had run dry, or had a leak in the system. A central heating engineer suggested this: That gas can be generated from the electro-chemical reactions of the various materials in a brake's hydraulic system, including corrosion, and are accelerated by the brake system's heat cycling. How otherwise can brake pedals sometimes go soft during use?
Brake fluid boiling during hard braking will definitely do it, and water contaminated or "wet" brake fluid will boil at 135-140F lower temperature than dry, new fluid.
Until a few year ago, I would agree that air (gas) could only get into the hydraulic system if the master cylinder reservoir had run dry, or had a leak in the system. A central heating engineer suggested this: That gas can be generated from the electro-chemical reactions of the various materials in a brake's hydraulic system, including corrosion, and are accelerated by the brake system's heat cycling. How otherwise can brake pedals sometimes go soft during use?
Brake fluid boiling during hard braking will definitely do it, and water contaminated or "wet" brake fluid will boil at 135-140F lower temperature than dry, new fluid.
Originally Posted by mk378
When the ABS activates the air will get out into the regular braking circuit. This can happen in "normal driving." There is a certain bump about 20 feet from a stop sign on my commute that will trigger ABS every time.
Ah, that makes sense. When activated, the ABS becomes the source of contamination to the regular braking circuit.
In this case, you might actually get a firm brake pedal following a standard bleed. And then after driving a bit and activating the ABS now and then, the pedal will go soft again.
When the ABS activates the air will get out into the regular braking circuit. This can happen in "normal driving." There is a certain bump about 20 feet from a stop sign on my commute that will trigger ABS every time.
Ah, that makes sense. When activated, the ABS becomes the source of contamination to the regular braking circuit.
In this case, you might actually get a firm brake pedal following a standard bleed. And then after driving a bit and activating the ABS now and then, the pedal will go soft again.
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