Thread: Sinking brake pedal fix. Dodgy as though

they are accumulators, they provide the dampening function as part of the pedal pulse thing and releasing the brakes when the abs is doing its bit. They are critical to ABS function as without them if you kept your foot jammed on the brake the fluid would have nowhere to go when the abs system needs to back off the brake line pressure to get the wheel to free up and turn again.

Yep, just been studying the hydraulic schematic. Took a bit of time to get my head around.

Ill do the blanking out and test it tomorrow. Hopefully this will narrow down the cause.

Looking like a leaky outlet solonoid valve, or internally leaky shuttle valve.

Either way i see a new modulator in my very near future.

Originally Posted by Blknight.aus

they are accumulators, they provide the dampening function as part of the pedal pulse thing and releasing the brakes when the abs is doing its bit. They are critical to ABS function as without them if you kept your foot jammed on the brake the fluid would have nowhere to go when the abs system needs to back off the brake line pressure to get the wheel to free up and turn again.

its the shuttle valve.

I bet if you pump the pedal up 3 times it stays rock solid. if it doesnt the fauly may not be in the brake module but in the MC itself which is recirculating through its fast fill port into the reservior.

I can pump a million times and it slowly bleeds down to the floor, after going solid.

I can blank of the master, its rock solid no sinking. Brand new also... not that means much.

Anyway, further looking at the schematic. This whole abs setup is dodgy as hell.
There is a direct path to the accumulators from the master via 3 check valves - not opposing flow. The setup is designed to allow pedal sink!!!!!!!!!!!!!!!

Now to convince the roadworthy inspector.
Dumbest setup ever.

An abs delete is in my future

Originally Posted by Blknight.aus

its the shuttle valve.

I bet if you pump the pedal up 3 times it stays rock solid. if it doesnt the fauly may not be in the brake module but in the MC itself which is recirculating through its fast fill port into the reservior.

Originally Posted by Discosux

I can pump a million times and it slowly bleeds down to the floor, after going solid.

I can blank of the master, its rock solid no sinking. Brand new also... not that means much.

Anyway, further looking at the schematic. This whole abs setup is dodgy as hell.
There is a direct path to the accumulators from the master via 3 check valves - not opposing flow. The setup is designed to allow pedal sink!!!!!!!!!!!!!!!

Now to convince the roadworthy inspector.
Dumbest setup ever.

An abs delete is in my future

maybe just use all that misguided blaming of the system to simply bleed the air out of it ( including the modulator)

No misguided blaming. See the pic.
If anyone can explain how this setup does not by design allow pedal sink.

Originally Posted by discorevy

maybe just use all that misguided blaming of the system to simply bleed the air out of it ( including the modulator)

Hi mate,
Im sooo not sure about this now.

Bloody deep diving into Wacko ABS design here

When the pump is actuated, B line is pressurised. This will fill the accu' chamber. This fluid will not release due to the normally close outlet valves (11) and the check valve (9).

So in a perfectly healthy system the accu's are maxed out and stay that way, as long as there has been a pump event, some time in the past.

With the accumulators maxed out = no sinking brake pedal.


Leaky outlet valves (11) cause a sinking pedal because they allow the accumulators to return to their designed home position.


Off to the shops i go, Looks like I have stuffed / leaky outlet valves.

Hopefully just O rings.

Originally Posted by Blknight.aus

they are accumulators, they provide the dampening function as part of the pedal pulse thing and releasing the brakes when the abs is doing its bit. They are critical to ABS function as without them if you kept your foot jammed on the brake the fluid would have nowhere to go when the abs system needs to back off the brake line pressure to get the wheel to free up and turn again.

In a healthy system the fluid circulates through the red lines(both sides) when the pedal is pressed, the other circuits are for active modes when the return pump is activated, on all D2s the brake pedal is sinking a bit if you keep pressure on it stationary, that's how they behave while the braking action in movement is not afected, if your pedal sinks too much without having a fluid leak you need a modulator power bleed in the first place, it's exagerated to blame the modulator without that...

An abs delete is in my future

that means no TC, EBD or HDC either