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Old 01-21-2019, 07:13 AM   #1
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Exclamation Weird brake problem

My pedal is soft. It travels more than what I would call normal before you can feel the brakes start to grab

When the car is off (no boost) the pedal is rock hard
I have replaced the master cylinder and the system has been flushed and bled twice, so I an very confident that there is no air in the system. ABS works properly. The ABS pump has been tested with a PWIS and is fine

The brake lines are newer and SS. Pads and rotors are newer

When the car is running and you have brake boost the pedal is soft.

Our theory is there is too much boost.
Could that be the case?
Can a booster fail in such a way that it increases boost?

I need to get the wrapped up fast as I am planning on going to the track this week

I guess I could always disconnect the booster and go with manual brakes

Your suggestions are very welcomed
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Old 01-21-2019, 07:18 AM   #2
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That sure sounds like a bad m/c. Did you bench bleed it before installing it?

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Old 01-21-2019, 07:57 AM   #3
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booster should not change engagement point, just effort. if engagement point is changing, either (a) your fluid is compressing (air in system) (b) the caliper piston has to travel further (thin pads) or (c) your fluid is going somewhere (leak in the caliper at the bleed valve or piston, bad MC, bad ABS pump).

given system is bled, pads are newer, MC is new (unless you got unlucky w a bad new part) and no obvious signs of leakage, suggest ABS pump may be culprit? realise it could be failing without producing a code (i think abs codes and piwis tests are more for proper actuation of valves, and not for leaks). i'm no brake expert, however.
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Old 01-21-2019, 08:14 AM   #4
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More info

If I disconnect the booster and drive, the pedal is rock hard, just have to push harder

No leaks we can detect.

How would the ABS pump effect the pedal if ABS is not engaged?
My understanding is the ABS system only has an effect when engaged, otherwise the just passes fluid
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Old 01-21-2019, 08:31 AM   #5
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here is a site that does a good job of illustrating the inner workings of an abs pump:

https://www.brakeandfrontend.com/abs-esc-hydraulic-control-unit-modulator-diagnostics/

you can see that an improperly seated/leaking valve would give the fluid a place to go that isn't the caliper and cause the pedal to travel further before it engaged.

i guess first step would be to determine if your issue is engagement point (i have to push pedal further) or effort (pedal is too easy/hard to push).

a quick search for symptoms of failing abs pump indicated that progressively non-responsive brake pedal is one of them.

and hey, it's the one component you haven't replaced yet (because it's expensive). if you do change it, note that there are multiple versions based on year and types of traction control, so double check part #'s.
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Old 01-21-2019, 08:55 AM   #6
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To this end, I recall reading (maybe in 101 projects? ) a specific procedure for bleeding the system when air is in the ABS system.

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Old 01-21-2019, 09:31 AM   #7
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Quote:
Originally Posted by The Radium King View Post
here is a site that does a good job of illustrating the inner workings of an abs pump:

https://www.brakeandfrontend.com/abs-esc-hydraulic-control-unit-modulator-diagnostics/

you can see that an improperly seated/leaking valve would give the fluid a place to go that isn't the caliper and cause the pedal to travel further before it engaged.

i guess first step would be to determine if your issue is engagement point (i have to push pedal further) or effort (pedal is too easy/hard to push).

a quick search for symptoms of failing abs pump indicated that progressively non-responsive brake pedal is one of them.

and hey, it's the one component you haven't replaced yet (because it's expensive). if you do change it, note that there are multiple versions based on year and types of traction control, so double check part #'s.

That's a thought.

Lets say the ABS pump is leaking, where would the fluid go?
I see any nor is the level dropping

Also, when the vacuum booster is disconnected, the the pedal is rock hard, just have to push like a M-Fkr

The brakes do work and so does ABS. Its just the pedal is soft, it travels about halfway before you feel braking.
In speaking with one of my track-rat friends, he suggested changing the pads.

In reading symptoms of failing ABS module, the pedal would get harder, not softer
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Last edited by JayG; 01-21-2019 at 09:35 AM.
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