Brake Pedal Slowly Sinking When Braking

[Peezen]

I have question for the community as to what my next step should be. I will explain what I have done so far and why.

One day coming out from work my brake pedal was super hard when braking, but only when the X5 was really slow. Braking was normal at higher speeds and then when close to stop the pedal would get super hard and I would essentially be manually stoping the vehicle.

After researching it seemed like it was a brake booster issue, either from the pump, check valve, or the booster itself.

I was about to embark on changing the alternator bracket gasket anyways, so I added this task to the list. Since I had just recently replaced the vacuum pump I crossed that off the list as a problem and decided to go with the check valve first.

I disconnected the driver side wheel speed sensor, and did the alternator bracket job (convoluted job for sure) then I replaced the front brake pads while putting the front axles back together and made sure to start compressing the pistons with the MC cap of. All seemed good.

I then moved to the booster job where I replaced the check valve and tested the breaks, but that didn’t fix the issue. I figured the brake booster then must be the issue and went about replacing it. I took the MC off and moved it out of the way without disconnecting the brake lines and replaced the booster with a Cardone reman. When I replaced the MC and tested the brakes the hard pedal was gone. Except now what I was getting was a normal feeling boosted brake that would slowly sink to the floor if I kept the pressure on.

I now have the brake light trifecta on for the front driver side wheel speed sensor, according to the reader, and a brake pedal that slowly sinks when pressure is consistently applied. The brakes work yet the pedal continues to sink.

I can get a new wheel speed sensor since maybe it was damaged, but what would cause the new braking issue?

I didn’t open the brake system at all, but could air have found its way in while I was moving the reservoir and MC out of the way of the booster? Could the internal seals have blown when I compressed the calliper pistons?

The fact that there seems to be boost for the brakes would indicate that the boost side is now repaired correct?

I just want to know what to do next. I do have the Scwabben Foxwell BMW specific reader and it does have a bleed service that I could run to bleed if air was introduced, or I could have a shop do it, or I could source a new or used MC and install it and then do the bleed at home or shop.

Any advice with my situation would be greatly appreciated. Love that it doesn’t leak oil anymore, not stoked that now the brakes don’t work.

[andrewwynn]

Two steps for abs bleed on foxwell. I forget which and which order


Sent from my iPhone using Tapatalk Pro

[Attacking Mid]

After doing a brake job on one end of my E53 and then doing a full 4-wheel pressure bleed, I could not get a firm pedal despite having a firm pedal before starting the whole process. I ended up going through like 2 liters of brake fluid, activating the ABS pump multiple times using ISTA, all to no avail. I now have a new master cylinder sitting on the shelf waiting for me to have time to install it. I'm hopeful that will bring my system back to proper operation. I chose that path based on searching through forum posts with several people commenting that the MC was their source of a soft pedal.

AM.

[workingonit]

Originally Posted by Attacking Mid

After doing a brake job on one end of my E53 and then doing a full 4-wheel pressure bleed, I could not get a firm pedal despite having a firm pedal before starting the whole process. I ended up going through like 2 liters of brake fluid, activating the ABS pump multiple times using ISTA, all to no avail. I now have a new master cylinder sitting on the shelf waiting for me to have time to install it. I'm hopeful that will bring my system back to proper operation. I chose that path based on searching through forum posts with several people commenting that the MC was their source of a soft pedal.

AM.

Not on my E53, but I had a similar situation on my wife's Cobalt: new pads & rotors, adjusted rear shoes & E-brake/parking brake, and used my Foxwell to bleed the ABS. No success, so I replaced the master cylinder, and used 2 quarts thru my power bleeder, and repeated the adjustments and bleeding at all 4 wheels, and used the Foxwell-ABS bleeding procedure again.

Still had a very slowly-sinking pedal...good enough for me to drive, but not for my wife (she has trouble braking/breaking! everything). I finally gave up, and sold the Cobalt 2 weeks ago (after three years of sporadically working on the brake problem).

To this day, I'm wondering whether or not the Foxwell NT510 Elite pressure bleeding procedure worked, and will it work?, if I ever need to do the same on my E53, my other GM vehicles, or even on the new-to-me Lexus that replaced the Cobalt.

I'm about to buy the Toyota-Lexus-Scion software for my 510 (I bought it with GM software, and added BMW-Mini-Rolls Royce later on), and would like to know if the ABS bleed actually works, or should I buy an "ABS bleeding-specific" scantool for that job, in the future?

[Peezen]

So I have ordered a new MC to instal as that seems to be the major reason for the pedal being able to sink all the way to the floor.

If I pump the pedal a few times it will build pressure back up, but if I add sustained medium pressure it will continue to sink. Does this sound like air somehow got into the MC when I manipulated it out of the booster, or does it sound like the booster could be the issue.

I should add that when I originally installed the MC into the booster I missed the the pin in the booster. I started the car and tested the brakes and was shocked that ere were no brakes at all, had to use the parking brake to stop the heavy beast.

I opened it back up and noticed that I had missed the getting the pin inside the MC. Could this have somehow caused damage to the booster or MC?

I’m a bit stumped right now as the problem was originally a hard pedal and now I have a soft pedal…

[Secretasiansam]

I have the same symptoms. My research points to booster leak. I can hear a whoosh of air when I press the brakes.
I have a booster and Mc awaiting install.

[Peezen]

Originally Posted by Secretasiansam

I have the same symptoms. My research points to booster leak. I can hear a whoosh of air when I press the brakes.
I have a booster and Mc awaiting install.

So the Brake booster could be responsible for the hard pedal AND soft pedal?

I had hard pedal before installing a reman booster and now I have soft pedal.

[stiubhartach]

The pedal slowly sinking to the floor is brake fluid leaking around the piston in the master cylinder. The fix is to replace the master cylinder. It’s not dangerous, just annoying.

A soft pedal feel or a spongy pedal is from air in the system. Anytime the hydraulic system is opened, you’ll introduce air into the system. Air is compressible while fluid is not. You’re feeling the air bubbles compress. This is not dangerous if there’s not a lot of air, just annoying and reduces the braking quality. Air can be in the slave cylinders or in the abs maze. The fix is to run the purge program using a computer tool.

It sounds like you had multiple brake issues and you fixed the booster issue. Now you have to fix the other two issues.

[Peezen]

Thank you so much for the input. I have a master cylinder on the way and will install it. I have the BMW specific Foxwell reader, any specific way to bleed the ABS? Do I still prime the MC with these cars?

[andrewwynn]

I'd look for the tis outline. It will describe the steps using bmw computer software but the method can be reproduced with foxwell


Sent from my iPhone using Tapatalk Pro