E53 X5 4.8is Pedal Soft and Spongy after BrakeBooster Replacement

[4799cc]

My brake booster died recently, so I ended up taking the master cyl / booster out and replacing it. BEFORE I started work on the system, the pedal felt fine - there was just no power assist (It took all my might to bring the brake pedal to stop the car - felt like I was trying to compress wood). It turns out the cause of the booster failure issue was my vacuum check valve failed, then engine oil (from the vac pump) leaking into the booster causing the seals to fail.

Anyway - I have definitely introduced air in to the system during this process since I had to open the system from the "top" (I had to undo the 2 brake pipes that run from the master cyl into the DSC pump).

I have now gotten everything back together. I put my master cyl back on, filled it and went to bleed it. I first tried the 2-man brake bleeding method wherein one person presses the brake pedal 5 times, and on the 6th one holds the pedal down while the 2nd man cracks the bleeder and watches for bubbles). With this method I got a ton of air out of the rear right caliper bleed nipple. I then moved to the rear left, then up to the front right and finally to the front left, each time bleeding until bubbles stopped and then continuing the process until I got clean fluid. I estimate that between my helper and I we'd have done close to 3-400 pedal presses - up to 100 per corner - but no firmness whatsoever.

I then did some reading and saw that I have to activate the ABS/DSC pump since that will likely have air in it due to opening up the braking system. I have working ISTA-D which I used to activate the DSC pump. I had my helper pump the pedal 10 times, hold it down on the 11th then "trigger" the DSC pump. I then opened up the bleed screw at the rear right caliper. All I got was clear brake fluid - no air bubbles. I repeated this process on the rear left then up to the front right and front left, each time getting no air just plain brake fluid. At this point I'd have bled out about 1 liter (0.5 qts) of brake fluid through the system.

Now I STILL have no pressure AT ALL in the pedal - it feels the same as when I started.

Another odd thing is the brake pedal feel isn't uniform all the way to the bottom. It gets "Easier" to press the last 40% or so of the way. Is this a bad thing, or is it a case of the piston only ever moving the first 60% of the way through the life of the vehicle and now I'm forcing the poor thing all the way to the bottom? I have no idea.

Is my master cylinder dead or do I just need to persevere with the bleeding? If its is indeed dead, what killed it?

[AceEngineer]

You may have to take it to a BMW mechanic. The manual talks about air pockets in the ABS module requiring a special pressure bleeder that pulses the pressure at some high rate to break loose the tiny air bubbles that can cling to the valve edges. Are you using the low viscosity brake fluid that meets BMW specs? I would continue to try bleeding a bit more before I take it to the BMW guys.


AceEngineer