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Well there's the check valve, grommet, master cylinder gasket, and the pedal plunger/piston. Those are all places the vacuum could leak out from. I suppose also the booster housing itself, where the seam is but that's a static seal. I could not find a new booster anywhere. I got lucky when someone here sent me an eBay link to a used one. After taking my old one apart I think it is still fine. There really isn't much to go wrong in there or wear out.
What I am wondering at this point is if brake fluid leakage from the master cylinder is causing the rubber to become more porous and allow the system to bleed down over time, but not bleed so fast it impacts function when engine vacuum is present. I'm hoping to learn more when the parts show up.
My guess would be one side was loose and put side load on the other causing it to shear off.
@NCRider Evidence suggests this is exactly what happened. The passenger-side stud was sheared, the driver-side stud was missing a nut. (I searched but never found it on engine splash shields.) I had originally thought both sides were sheared.
Photo of old booster.
My lesson learned is to check the nuts on the studs that hold the master cylinder to the booster if I ever have a squeak or "catch" in the brake pedal again. I never imagined that these nuts might back off. The dealer looked into my brake pedal complaint, but evidently missed the loose or missing nut also. They recommended replacing the booster, a diagnosis which I did not think to be credible. To be fair to everyone I had look at this, it requires a two-person diagnosis, one person to press on the brake pedal, the other to observe the booster.
As one might expect, the car's brakes are so much tighter and better with the new booster. Brakes engage at very top of pedal travel. No more intermittent squeak in brake pedal. Pleased with the result.
New and tightened nut on passenger side. New and tightened nut on driver side.
@Joe Z Yeah. But I am very grateful it did not fail during full send on a canyon road or a track.
That could have been really bad. Hopefully this remains a rare occurrence. The probability of difficult repairs is why I bought the ISF instead of an E90/E92.
Last edited by ChpEng; 09-26-21 at 10:39 PM.
Reason: added tag for Joe Z
Brakes almost always feel better after they've been bled. It's a lot of the reason guys say their brakes feel better after installing braided stainless lines. It's not the lines, it's just the improvement from a decent bleed and fresh fluid.
Brakes almost always feel better after they've been bled. It's a lot of the reason guys say their brakes feel better after installing braided stainless lines. It's not the lines, it's just the improvement from a decent bleed and fresh fluid.
I agree in principle and have experienced same.
In this instance, I had the rotors, pads, lines replaced and fluid flushed in late April. Braking is so remarkably different and better now, that my assessment is that either the master cylinder was tilting against one nut, or the original booster was faulty, or both.
09-16-21, 08:28 PM
