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Discussion Starter · #1 ·
06 LR3 4.4 82k miles stock.

On 99% of normal riding the brakes work just fine and the vehicle stops as expected but if I hit the pedal really quick like in an emergency situation, The pedal feels hard.
I don't believe there is a leak in the vacuum otherwise it would always be hard.
I tried pressing multiple times on the pedal with the engine off and it always starts breaking at the same place, meaning the the pedal does not become soft.
I consider the possibility of some issue with the booster vacuum pump but I can't tell if it runs. I am not sure when and how often it is supposed to turn on.

The problem seems to be dependent on how quickly I slam on the pedal. I can be slowing down really fast at a stop and it does work fine but if I need to stop very quickly and I hit the brakes in a fraction of a second then it is hard and it stays hard until I release it and press it again.

Any idea what the problem might be? also how do I test for proper vacuum and pump working?

Thanks!
 

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It still stops fine though, right?

One of the several subsystems in the ABS is Emergency Brake Assist. EBA increases hydraulic pressure in the system to maximum braking limit, based on traction available, via the precharge pump in the modulator. This makes the pedal feel hard, as there is already pressure in the system that you are pushing against.
 

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Discussion Starter · #3 ·
It still stops fine though, right?

One of the several subsystems in the ABS is Emergency Brake Assist. EBA increases hydraulic pressure in the system to maximum braking limit, based on traction available, via the precharge pump in the modulator. This makes the pedal feel hard, as there is already pressure in the system that you are pushing against.
Good point. My problem could definitely be happening when the system detects a "panic stop" but unfortunately it doesn't look like EBA is applying any extra braking pressure. As a matter of fact in those situations the ABS never kicks-in unless I put all my weight on the pedal.
If I do a normal stop i can easily get the wheels to lock and then I feel the ABS modulating the brakes, but in a panic stop the wheels do not lock and the stopping distance seems longer than in a normal quick stop.
 

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Braking distance is increased with ABS activation. EBA does not apply full pressure, just maximum pressure per available traction, so essentially to the limit of ABS activation.
 

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Discussion Starter · #5 ·
As far as I know EBA is supposed to apply full braking power when a panic is detected, then it's up to ABS to modulate the baking to make sure wheels do not lockup.
EBA does not know anything about available traction, ABS does.

My problem is that in a panic brake it takes twice as long to stop than in a non-panic situation and that's dangerous.

Is there a way for me to test if the vacuum pump works?
 

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Vacuum pump is not the issue, as it is essentially a back up to engine vacuum.

EBA is a function of the ABS, so yes, it does know exactly what is going on.

It taking longer to stop is an issue however, which means that the pump may not be delivering the correct 'maximum' pressure, which could be anything from poor fluid condition, to a weak pump, to a bad seal somewhere in the system.

If this is only happening during very quick pedal depression, then it is due to EBA kicking in, but again, it should not increase stopping distance.
 

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Discussion Starter · #9 ·
Bringing this thread alive again as I am still experiencing the same problem.
Since my last post I replaced rotors, pads and fluid.
Normal every day braking has slightly improved but emergency braking has not.
If I slam on the brake the pedal is really stiff and the car barely slows down but if I depress the pedal down at a normal speed then the the LR3 stops quickly.

Does anyone have a brake pump schematic that I can look at? I am starting to think that if the system has two pumps, one activated by the pedal and the other by the EBA/ABS, could it be that the latter might have air in the fluid? Being sort of a separate circuit it might be difficult to properly flush the fluid.

I've heard that LR has a procedure that uses the testbook to flush brake fluid, does anyone know what does it do differently than just manual flushing?
 
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