Gents,

I wanted to maximize air bubbles bleeding, so I look at into W212 ( facelift ) braking system in depth.
Its not called ABS per se anymore but now it is ABR ( adaptive brake ) and ESP ( electronic stability).
ABS is just a small function of the overall complex function.
I attached the zip for you to learn of it.

The main goal is I was trying to understand how the ABS/ESP hydraulic module fluid gallery can store/hide air bubbles.

The ABS/ESP system supposedly is a total bypass or able to be by-passed and only starts to take over to provide maximum threshold precision for braking system when and if the need arises...well it is NOT TRUE.
When one goes to Xentry menu and go to N30/4 ESP ( no ABS menu, it is called ESP ) while engine is running, the instrument cluster will warm that ESP is not active and in DIAGNOSTIC mode.
Here one can try to move the car in a safe area and play with the brakes, one will find that the brakes will be much less braking power.
The brake booster is pure mechanical unit, and the vacuum pump that powers it..... is a mechanical driven by camshaft and can not be turned off, so it is not the lost of brake booster assist causing braking power decrease.
So, the only way we are to get braking power decrease is when the brake fluid flow is somehow manipulated or diverted or whatnot, this is why I said it is not a total by-pass system.


Upon reading deeper into the tech documents, some interesting features exist and this can be the reason why someone may find the brakes kind of "mushy or spongy" in a calm driving mode.

Among others.......... open the document : ABR - Adaptive Brake , function.pdf
- Dry braking function sequence - in the rain the system will dry the brake pad+rotor once in a while without you knowing. The ESP talks to rain sensor and the wiper too.
- Function sequence for precharging depending on accelerator pedal actuation.
- Function sequence for precharging depending on lateral acceleration




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- BAS function sequence , this I believe is the one giving the mild spongy-mushy feeling.




To me, I will explain my best for BAS , this way :
If say the ABS/ESP module is a total bypass for our brake fluid when not in emergency, how we modulate our brake pedal won't change, except if brake booster is out of vacuum pressure.
What I sense is, at slow speed or crawling speed and in a driving pattern not deemed agressive by the ESP/ABS module, I am given a very high latitude of pedal modulation range....aka deep pedal distance of travel.
Back to my Bali villa sloping parking space, the final slope is steep for parking area to road. See below.



Here if I use my cheapo Toyota which has ABS & ESP too, but not as intelligent as the W212, to modulate the brake to crawl slowly centimeters by centimeters and not feel the sensation of braking or kind of sudden stop, IS NOT POSSIBLE.
On the W212, I can like glide down so slowly accurately, since the brake pedal depth is deep , its a wide band pressure capable, but in very small minor increment.
Imagine you need say 100kg of force to stop the car. The cheapo Toyota pedal output is 0 - 20 - 40 - 65 - 80 -100 kg or 20 kg increment, the W212 can produce increment as though 1 to 100 kg in 1 or 2 kg increment, very refined.
I hope my explanation makes sense.

Now, this kind of pedal feel with deep travel can cause us after a brake job needing full air bleeding and have done so, to assume the brake pedal still is air bubble effected, while it is not.


If one reads the BAS information carefully, here is the clue that indeed ABS/ESP module is like rationing power of the brakes.
BAS detects emergency braking situations from rapid actuation of the
brake pedal and, if necessary, increases the brake pressure in order to
achieve maximum possible deceleration.
Our leg pedal pressure with vacuum booster assist and all the multiplication of force by pedal travel and its hinge point....and size of master cylinder and the size of caliper pistons, is a value more than enough to generate maximum
brake pressure for threshold braking without locking up the wheel or locking up the wheel.
Accuracy wise for not going into a lock up mode, yes the ABS module is far superior compared to even the best brake pedal modulator in the world.
In order to have room for pressure increase, that means the pressure in the first place is limited on purpose. Don't think the pump at the ABS is for boosting pedal pressure, it is not for that.
Our leg is still the pressure decider mechanism, MB does some intervention to make the pressure increments smooth.


I can't find W212 brake hydraulic circuit, I can only find W210 one. W211 SBC is a different animal, so we can't use that as reference.

From W210 era. ABR of W210 era - Traction system hydraulic unit, function.pdf



I had some initial worry that I may have air bubble at the item 11, accumulator. Its probably a small cavity only, but even 3 air bubbles of bleeder hose size can wreak havoc.
Hence I wanted to know all brake fluid gallery/path.










Our W212 uses the 12 solenoids type ABS, not 8. Its more refined the 12 valve solenoids.


Okey dokey, that's it........so when doing brake air bleeding, at least on W212 facelift ( I am sure non facelift is the same ) , do not get fooled by the sensation of "deep brake pedal" MB engineered our brakes to feel like when driven calm.
All you need is to push brake pedal a tiny bit harder.

 

The attachment........

 

Thank you Surya for this great ABS/ESP study.
It's very impressive to see the details of how Bosch has perfected the ABS control to deliver additional smarts built on sensor data and software.

The superbe ECU design manages powerplant as we came to appreciate.
Bosch did not just stand on that success, they injected a lot of more intelligence in the ESP to manage brakes, electric steering, distances....