is my master brake cylinder went bad?

[boneca]

Hi guys, I don't post much in General section as I've been somewhat lucky with my car in the past 4 years.

Lately I don't drive it much as I got a daily few month ago but I still drive it at least once a week to put gas, wash, going to meets and buy coffee



well what they say "car breaks more when u don't drive it" apparently true


anyway couple of weeks ago on Saturday I took my car out and my breaks felt wrong, they were too soft, I pumped the pedal drove little bit and it seemed back to normal, I left it at that

last Saturday I took my car for a drive and right away breaks felt the same but this time around I wasn't able to pump it, it was still soft.

I parked my car in the driveway and decided to engage electronic brakes which I don't recall doing ever before, sure enough I wasn't able to disengage them, the system won't recognize that break pedal is pressed, I had to manual disengage my parking break using the hook inside the trunk. I got a message about the breaks right after but I'm sure it will go away once the issue is fixed










there were no other errors at all even after connecting the scanner


I started checking:
1. The level of break fluid is on Max
2. no signs of break fluid anywhere under or near the car
3. most likely there is no air in the system not only because of above but also its been a while since the break fluid, rotors and pads were changed, no bubbles in the break fluid reservoir when pedal pressed
4. the calipers are dry
5. the master break cylinder looks like it was just purchased (that clean)





I started to look for similar issues online and it seems everything is pointing to the master break cylinder seals, the other option is break booster but the symptoms there are different (the pedal becomes harder to press not softer like in my case)

my mechanic came over and we looked at everything again also spent some time to cleaning the clogged seal under the booster to let bunch of water to escape, also stumbled upon this black box, it was half covered in water, what is that, alarm?





looks like my mechanic not convinced its a master break cylinder issue even though everything points in to that direction.

If anybody has any ideas or experience with similar behavior I'd like to hear it!



Thank you all!

[yco]

it can be brake fluid, air in the brake lines, bad calipers etc also.. when was the last time you ve changed the brake fluid?

[boneca]

Originally Posted by yco

it can be brake fluid, air in the brake lines, bad calipers etc also.. when was the last time you ve changed the brake fluid?

I appreciate your reply, you probably didn't read my post entirely, I already rolled out the causes you mentioned.

Last time break fluid was changed couple of years ago but it's too early to change it again according to time and mileage.

Rotors and pads were changed last year, I didn't change the break lines but they were checked and no issues found!

Thank you!

[TwinSnailz]

Black box is your beep and beep beep lock confirmation. Oh yeah and siren alarm speaker.

[boneca]

Originally Posted by TwinSnailz

Black box is your beep and beep beep lock confirmation. Oh yeah and siren alarm speaker.

I appreciate it!

this is what I thought as well, ty for confirmation

[jandref321]

Hey Boneca, When did you last swap brake lines? They could be getting old and flexing more, or the center rubber lines above your rear differential could be leaking slightly...those drip onto the diff and you wouldn’t see a puddle. It could still be your master brake cylinder diaphragm as you described, but if you haven’t replaced your brake lines I would do that first for preventative maintenance and then bleed the lines with power bleeder and scan tool to activate abs. If that doesn’t work then go after the master cylinder.

UUC and I believe turner and ECS make exact fit lines. The center pair above the diff is a pain...I still haven’t figured out a way to swap those without dropping the exhaust and differential.

Last note: check the vacuum lines for the brake booster? You said the brakes feel soft...is the pedal hard though?

[boneca]

Originally Posted by jandref321

Hey Boneca, When did you last swap brake lines? They could be getting old and flexing more, or the center rubber lines above your rear differential could be leaking slightly...those drip onto the diff and you wouldn’t see a puddle. It could still be your master brake cylinder diaphragm as you described, but if you haven’t replaced your brake lines I would do that first for preventative maintenance and then bleed the lines with power bleeder and scan tool to activate abs. If that doesn’t work then go after the master cylinder.

UUC and I believe turner and ECS make exact fit lines. The center pair above the diff is a pain...I still haven’t figured out a way to swap those without dropping the exhaust and differential.

Last note: check the vacuum lines for the brake booster? You said the brakes feel soft...is the pedal hard though?

Thank you for suggestions! I don't believe the break lines were ever replaced on this car, it would be a good idea to replace them but it seems to me its easier to start with the master cylinder just because of the access, sure if it won't help then it would no brainier to go for new lines!

The pedal is soft that's why I'm leaning more towards master cylinder vs. the booster.

[philly42]

Soon after getting an X5M with 70k, I started to notice an issue with the brakes. Essentially I ended up replacing the rotors, pads, bled the system myself, then had the dealer bleed it. With still having issues, I measured the vacuum going to the brake booster and verified the check valve was good.

Sounds to me that you are at the point when I decided to have the brake booster/master cylinder replaced. I had a BMW mechanic replace the booster and master cylinder since it had to be removed to get the booster out. The mechanic confirmed that the booster was the issue.

To further test symptoms with the brakes, find a steep driveway or parking garage ramp. Stop while driving down. Then ease off the brake a little (keep some pressure) to have it roll about a foot or less then immediately press hard to stop. Do this a few times. If braking power an feel is not consistent or the car rolls a bit when you press hard, there might be an issue with the booster.

If you're having issues with your brakes in extreme situations...track event or trailer heavy load, then look into the brake lines. If your brake issues are with normal driving, the problem is likely with the master cylinder or booster. (considering the fluid is fresh with good pads and rotors)

[boneca]

Originally Posted by philly42

Soon after getting an X5M with 70k, I started to notice an issue with the brakes. Essentially I ended up replacing the rotors, pads, bled the system myself, then had the dealer bleed it. With still having issues, I measured the vacuum going to the brake booster and verified the check valve was good.

Sounds to me that you are at the point when I decided to have the brake booster/master cylinder replaced. I had a BMW mechanic replace the booster and master cylinder since it had to be removed to get the booster out. The mechanic confirmed that the booster was the issue.

To further test symptoms with the brakes, find a steep driveway or parking garage ramp. Stop while driving down. Then ease off the brake a little (keep some pressure) to have it roll about a foot or less then immediately press hard to stop. If it's not consistent or rolls a bit when you press hard, there might be an issue with the booster.

good advice, I'll make sure to test it out

[Ryan5034]

Have you checked the brake pedal sensor under the brake pedal? Should be a sensor down there somewhere.

[boneca]

Originally Posted by Ryan5034

Have you checked the brake pedal sensor under the brake pedal? Should be a sensor down there somewhere.

no I haven't, ty for the tip!

[Dave00]

First off, love the car!!! I'm a stickler for grammar so please note it's BRAKES not BREAKS on your car. Now, easy way to check the Master Cylinder is look for leaking brake fluid in the Booster by unbolting the MC from the Booster. The seal on the plunger rod is usually the first to go and the best sign is leaking fluid into the Booster housing.

[boneca]

Originally Posted by Dave00

First off, love the car!!! I'm a stickler for grammar so please note it's BRAKES not BREAKS on your car. Now, easy way to check the Master Cylinder is look for leaking brake fluid in the Booster by unbolting the MC from the Booster. The seal on the plunger rod is usually the first to go and the best sign is leaking fluid into the Booster housing.

I appreciate the love and thanks for the grammar check, I'll try to get better at it

I still haven't ordered the new brakes master cylinder yet, kinda want to but also trying to get updates from my mechanic who's been MIA in a last few days.

I assume u won't see the seal leak without unscrewing the cylinder from the booster? everything is dry over there! I rather have new BMC ready to go when inspecting the elements!

I'm actually thinking to do this change myself but need to figure out exact procedure to make sure there is no air in the system!

[boneca]

little update on the subject, I finally ordered the master cylinder yesterday, it should arrive sometime next week (maybe Tuesday) I'll try to get my mechanic to replace it as soon as it arrives, so hopefully I'll be back on the road by next weekend!

I'll keep u all in the loop and I appreciate all of u suggestions and attention!

Have a great weekend!

[Cwint]

Hi, I have just done the front wheel bearings on my car and had to stomp on the breaks to hold the wheel while undoing the big nut, after this I started the car and pedal now goes slowly to the floor like the seal in master cylinder is leaking, or the check valve in the cylinder is leaking.
Boneca, did you get the mc fitted? And did this fix the issue?

Thanks Chris

[boneca]

Originally Posted by Cwint

Hi, I have just done the front wheel bearings on my car and had to stomp on the breaks to hold the wheel while undoing the big nut, after this I started the car and pedal now goes slowly to the floor like the seal in master cylinder is leaking, or the check valve in the cylinder is leaking.
Boneca, did you get the mc fitted? And did this fix the issue?

Thanks Chris

oh its been more than a year already - time flies! I don't even own this car anymore and its been six month since, but yes I ended up replacing the mbc and its fixed my issue.