denis550

Hi,
my brake pedal is very soft when my engine is on and the car is braking very poorly and inconsistently. If i pump the pedal then it becomes hard, but if i keep it depressed with a steady pressure it slowly sinks to the ground. When the engine is off the pedal feels normal (fairly hard when pumping it and it doesn't sink to the ground when holding it). The pads and discs were replaced a few months back.
So far I tried fixing it by:

• flushing the fluid
• checking the brake lines for leaks
• rebuilding all brake calipers with repair kits
• replacing the master brake cylinder
• replacing all flex hoses
• bleeding the system for hours if not days at this point

As my car is an MY07 it doesn't have the SBC system. I'm running out of ideas what to try next. The only parts that i haven't changed are the brake booster and the ABS/ESP unit.

Has anyone had a similar experience with the brake system on a w211? And is there any way to check if my brake booster or ABS unit are broken?

bmwpowere36m3

Fluid is moving somewhere, whether leaking externally or internally. Are you losing fluid in the reservoir during this or is it steady level?

Besides obvious, bleeding system including ABS pump…

Barring an external leak, one would suspect the MC.

denis550

The fluid level is steady all the time and I'm 99% sure that i don't have an external leak.

I'm gonna start by bleeding the ABS pump unit because I already changed the MC and nothing changed. What are the chances that a brand new MC has an internal leak out of box.

What's the correct procedure for bleeding the ABS pump? Do I just undo one brake line at a time and bleed it the same way like I do at the wheels? There are really contradicting stories on the internet. The ABS is self-bleeding, it is bleed by just driving the car and using brakes, do I need a STAR diagnostic to do it, and so on...

NewShockerGuy

I believe when I swapped out the calipers I had to pressurize the system to 1 bar. Which is 29si to properly bleed the system.

However, one thing to look at and there are several threads on the forum here is something called piston over return. It's quite common on MB. When I did the swap, the pedal was squishy and went to the floor but still braked.. If I pumped a couple times then the brake pedal was fine. What was happening was the pistons were being pulled in more than they needed to be. This just doesn't happen on the AMG brakes but can happen on a lot of MB.

Let me see if I can find the thread that greatly helped me.
Mushy brake pedal feel after brake pad replacement. - MBWorld.org Forums

Glyn does a great job explaining why this happens in the above thread.

-Nigel

denis550

I never heard of piston over return but this was exactly what was happening on my car. I could even visually see it happening when the brake was pressed and released. The gap between the disc and the pads was just 1mm when the brakes were released but I guess that's enough to screw up the pedal feel.
Now the brakes feel as good if not better the the ones on my newer car

Thank you so much!

NewShockerGuy

Yep that's exactly what happened to me! Had I never found that thread I would have been lost and super frustrated! Glad it helped you! and your brake pedal feels normal now. In all the years of me changing and bleeding brakes on our vehicles this was the very first time I had ever expierenced it. I didn't even know it was a thing. I was almost ready to start throwing parts at the problem, but super glad I didn't.

-Nigel

bmwpowere36m3

In 20 years of wrenching I’ve never had this over-return issue… I understand the principle and function of the square cut seal.

Though, pumping the brakes should cause them to firm up (taking up the gap).

I’ll add this to my memory bank…