Bleeding the brake master cylinder

[Flow]

Here is a picture of what I mean. Is that a rust risk ?
Or is the metal treated below the paint ?

[Flow]

Finally had time for a test drive today as the weather was not complete shit and I was home.
The brakes are now perfect. I haven't had this pedal feel in years on my car, there is good initial bite and the modulation is easy. I am super happy about it now.
Looking back at the entire situation, I strongly believe it was simply a brake master cylinder that was not bled correctly from the start, and the workshop that I asked to replace it years ago did not do a bench bleed of the master cylinder.
The booster might have had an issue too, but I did not notice any fluid inside.

The job was really messy but necessary.

Thank you everyone for the advices, especially about the bench bleeding part, I would never have thought of it, and it made all the difference in the end.


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[bigjae46]

Finally had time for a test drive today as the weather was not complete shit and I was home.
The brakes are now perfect. I haven't had this pedal feel in years on my car, there is good initial bite and the modulation is easy. I am super happy about it now.
Looking back at the entire situation, I strongly believe it was simply a brake master cylinder that was not bled correctly from the start, and the workshop that I asked to replace it years ago did not do a bench bleed of the master cylinder.
The booster might have had an issue too, but I did not notice any fluid inside.

The job was really messy but necessary.

Thank you everyone for the advices, especially about the bench bleeding part, I would never have thought of it, and it made all the difference in the end.


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Haha...I double checked and I DID warn you it would be messy! Glad to hear that it fixed your braking issue!

I don't remember removing the DSC unit, definitely removed the ECU and the plastic box. That looks like metal. I would clean it up really well, may need to spray a can or two of brake cleaner on it and then hit it with some rattle can primer.

These cars are over 20 years old now so anything rubber and original to the car is at the end of its useful life. We are all just getting free time at this point.

[CrookedCommie]

I'm betting the booster had an issue, not the master cylinder or air in the MC.

I'm not sure how you can bench bleed and install without introducing any air.

[Flow]

I'm betting the booster had an issue, not the master cylinder or air in the MC.

I'm not sure how you can bench bleed and install without introducing any air.

Maybe. I don't know.

As for not introducing air, the theory I applied was to have brake fluid in the tank. As long as it leaks/flows out, air does not come in.

[bigjae46]

I'm not sure how you can bench bleed and install without introducing any air.

Just don't let the reservoir run dry between removing the fittings for the bench bleed and installing the hard lines.

[sapote]

The booster might have had an issue too, but I did not notice any fluid inside.

How do you check inside the booster for fluid?

Btw, I forgot to mention that the 2 long studs on front of booster that for holding the master: air can leak through the stud to booster can interface and it relies on the flatness between master and booster sheet metal to seal. So I added a bead of RTV around the base of the studs to ensure a perfect seal. How do I know? Before install the new booster I make sure it's not leaking, by taping up the center hole (to master) and blow into the hole where the booster vacuum hose enter, then I detect the leak at the stud bases.

[sapote]

Finished replacing the brake master cylinder last night, and my god what a painful job to do seriously ! I am certain I did not do it in the right order, but getting the bleeder off and brake line connected with the master cylinder in the car is a major PITA, with brake fluid leaking all over the place for the hour+ I needed to get the threads of the lines to grab and then tighten everything. I seriously considered burning the car down and calling the insurance.

I wonder replacing both master + booster is tough or just replacing the booster with master still connected to the metal lines to the DSC. I did it this way to avoid doing the bleeding. There is no room to get the big booster out but I managed to hold the master using strings (not permanently bent the metal lines much) and wiggle the booster out. It was small leaking at the pedal side through the rod seal to cause DME lean codes. This seal does this magic: closes the air when pedal released and opens the air way when pedal pushed down. I don't know how this seal can last after thousands of braking strokes for over 20+ years. Most cars still with perfect working original booster.

[Kdubski]

Finally had time for a test drive today as the weather was not complete shit and I was home.
The brakes are now perfect. I haven't had this pedal feel in years on my car, there is good initial bite and the modulation is easy. I am super happy about it now.
Looking back at the entire situation, I strongly believe it was simply a brake master cylinder that was not bled correctly from the start, and the workshop that I asked to replace it years ago did not do a bench bleed of the master cylinder.
The booster might have had an issue too, but I did not notice any fluid inside.

The job was really messy but necessary.

Thank you everyone for the advices, especially about the bench bleeding part, I would never have thought of it, and it made all the difference in the end.


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Keep us updated! My sound returned roughly 300 miles post MC/booster replacement which was some 12,000 miles ago now. I just learned to live with it. I hope your issue was resolved because that is one of the worst jobs in my opinion that you should have to do on the m3.

[Flow]

Will do, but I don't think it'll return. The thing was just broken imo, and maybe you had bad luck and got a faulty replacement ?

[CrookedCommie]

Keep us updated! My sound returned roughly 300 miles post MC/booster replacement which was some 12,000 miles ago now. I just learned to live with it. I hope your issue was resolved because that is one of the worst jobs in my opinion that you should have to do on the m3.

Did you replace the booster with a new OEM one, or another used one?

[sapote]

Looking at the parts after the fact, I believe it was simply the master cylinder that needed a good bleed, as there was no fluid in the brake booster.

The noise in the video of post #4 cannot cause by a good booster; a rooster maybe

[Flow]

The noise in the video of post #4 cannot cause by a good booster; a rooster maybe

Hey, you know why the rooster is the symbol of France ?
Because it's the only bird that manages to sing with its feet deep in shit!