EDIT: I found a function on my Launch scan tool that allows me to release each wheel at the abs unit one at a time. Launch calls this a Mix Up test Brake Lines. I guess calling it ABS release would have been to advanced or obvious. I used this test while the brakes were locked on the front wheels, each wheel released when the Scan tool commanded it but re locked as soon as the tool finished the test on that one wheel. The residual pressure is in the system exactly as if someone is pushing the brake pedal but the rear brakes are not affected. Still trying to solve this one. I can crack either of the front wheel bleeds just enough to let out a short squirt and both front brakes release until the brake pedal is pressed hard again. Thanks

 

Anxious to hear about your efforts.

I just ordered a new master cylinder for my '13 x35i - along with a pressure bleeder. My symptom is a pedal that will sink to the floor with some foot pressure. I discovered this problem while flushing my brake fluid last week.

I am anxious about introducing air into the ABS. Although I have ISTA+ to perform the bleed procedure.

Good Luck!

 

Tom Cat thanks for your reply. I would bet with 95% accuracy that your master cylinder like mine is still fine. You have Air in the system. I have bled dozens of cars in the past few years and not had this issue but after much reading I did purchase a Motive Power Brake bleed tool. I will pressure bleed it next. I am assuming I have air somewhere causing my issues but with both of my master cylinders installed I had a very firm pedal and the very same residual hydraulic pressure on the front brakes while the rears are fine.
I even removed the brake pedal from the brake booster to make sure the booster is fully releasing. No change. I removed the vacuum from the brake booster, no change.
I will try unplugging the DSC but I don't expect that to make a change.

Note: I read this morning that if the computer senses low brake fluid it shuts off the fluid flow to the rear brakes which can make them hard or impossible to bleed. My 08 X5 has the fluid level sensor mounted on the Master Cylinder cap. I had this removed all through my bleed process. I am thinking the computer might have been fighting me while I was trying to bleed due to this.
I will look at making sure this sensor is reading that the master cylinder is full while bleeding. That may be as simple as holding the cap upside down so the float is in the Full position not the empty position or possibly connecting the wires with a paper clip to close that connection while bleeding with the cap off.

 

Mine is actually a different symptom - pedal sinking to the floor with car running and firm foot pressure. Had an eclipse with the same symptom, years ago...

Locking-up calipers may be a failed booster. Look here: Brake Booster Issues Thread

 

I have read that thread, my brake booster was just installed, with my brakes locked after hard pedal pressure my front brakes, both sides equally will not release. While they are locked I have disconnected the vacuum brake line from the booster, emptying it of all vacuum and the brakes were still locked.

 

Update, I found If I loosen the master cylinder mounting bolts where it bolts to the brake booster about 2 turns on each nut the lock up problem goes away. I do not remember the brake booster to master cylinder rod being adjustable? There was nothing in the instructions for this new booster or new master cylinder about the rod being adjustable?
FYI: I also tried removing the pin where the brake pedal attaches to the booster. That did not release the brakes. Only adding a little clearance between the master cylinder and the booster released them. Still working on WHY.

 

Hey!! that's some find!

Do you still have the original MC & Booster to compare?

 

No both parts are gone.

 

Follow up, I ordered in a new Booster today, by bleeding the vacuum off of the new booster the brakes would release. I am not yet positive this is the cause but it sure is looking like the new booster had an issue. What made me finally order the new booster was a simple test. I disconnected the vacuum from the booster, applied the brakes several times hard without the vacuum asssist and they did not lock. Then I started the engine without the vacuum line attached to the booster. Engine Running I applied the brakes hard several times. The front brakes were free and normal after application. I tightened the MC bolts up to the booster and without the vacuum attached I could apply the brakes without the front brakes locking up. Engine running I hooked up the vacuum line to the booster, the front brakes locked without me pressing the brake pedal. The booster applied the brake force.

I will update after the new 2nd booster is installed, that should be next weekend.
Mark

 

Your problem is the brake booster push rod adjustment.

You need to adjust the push rod where your brake booster push rod attaches to the master cylinder piston rod ( or less likely where your brake pedal connects to the brake booster push rod. )

You released the master cylinder’s mounting nuts, and it didn’t lock up, indicating the push rod needs to be adjusted (it is too long, therefore, pushing the master cylinder too much, and pushing hydraulic fluid to the brake lines, locking them up prematurely! )

Removing the vacuum assist eliminates the excessive push, and your fronts dont lock up. That just confirmed it!


You don’t need a new brake booster or brake master cylinder. You just need to adjust the brake booster push rod.

Good luck.


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The push rod in these is not adjustable. It is on other cars but not on this car.
I actually drove this car today with the vacuum line disconnected from the booster. It drove fine with no drag. Very hard pedal without the brake booster working but no drag. While backing out the driveway the brakes locked up again. I lifted the hood and found the vacuum line had jiggled itself back into place and applied vacuum to the booster. I moved it out of the way so I could not self connect and the brakes released and the car could be driven but without power assist.
I will put on the new booster when it arrives Friday or Saturday and report back.

 

fyi. My brakes came around after its MC replacement.

Bleeding them really took some effort. Did an old-school pump and flush, then a bench bleed for the new Master Cylinder using a syringe, then replaced the MC while leaving the lines and ports plugged with silicone plugs, then did a Power-Bleed using the Motive European Bleeder, performed an ABS Calibration in ISTA+, then jammed the brake pedal down overnight wedged against the seat.

The brakes feel very good, again.

sheesh.

Best of luck with yours, too!

 

Final Wrap: new booster came in today, installed it, quick bleed and brakes are perfect. Can't believe all the BS and testing just to determine the new Booster was bad. Glad it is resolved.

Actual bleeding of this system is no harder or different than bleeding a late model GM or Ford, each asks you to activate the ABS system and bleed it also. My Launch Scan Tool X431V+ did this bleed process on the BMW exactly correct. I did purchase the Motive European brake bleeder with the aluminum cap for the brake master cylinder rather than the lower priced unit with a plastic cap. Works very well. I did not mess with pumping it up, Brand new I drilled a hole and installed a tire Schrader valve to use for bleeding off pressure and I can attach my compressor hose with it set at a constant 20 PSI so no pumping is necessary and it is easy to bleed off pressure when done. This worked very well so I did not play around with the pressure setting, 20 psi work fine and BMW says 2 Bar max which is around 28 PSI I think? No leaks, no mess, job done.

My problem on this repair was not recognizing the booster was applying the brakes all by itself. If I had a do over I would have tested the booster right away by applying vacuum and checking to see if the brakes applied. I did that with this new install. I did use another A1-Cardone unit as a replacement and it is working perfectly. The booster is a very tight fit on install, no way it can be installed with the Master Cyl Attached. Hope my headache helps someone down the line. Thanks to those that offered advice along the way.
Mark

 

Final Wrap: new booster came in today, installed it, quick bleed and brakes are perfect. Can't believe all the BS and testing just to determine the new Booster was bad. Glad it is resolved.

I did purchase the Motive …. I did not mess with pumping it up, Brand new I drilled a hole and installed a tire Schrader valve to use for bleeding…
Mark

Glad your problem is fixed.
Thanks for the solution for the Motive tank! That pump is absolutely useless! tire Schrader valve sounds like a great idea! Ill do it too! It took me 10-15 minutes to get to 15 psi with its pump!!


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