Brake Issue

Scottaw · 08-06-2023, 02:12 PM

2013 128i with 64K miles, have had it for four years now. A few weeks ago I noticed the brake pedal lost some of it's feel, usually the brakes have been very good, strong and responsive. I replaced rotors/pads at 56K when I got it. The brake pedal now feels almost as if there is leakage across the master cylinder piston. The pedal will slowly sink with a constant pressure on it. I can release the pedal and the bite is immediately at top of the pedal again, but will begin to sink after a few seconds of constant or increased applied pedal force. I had received a promotional service coupon from my local dealer so i had them bleed the brakes, since that was supposed to be part of the 60,000 mile service. The first week or so after driving it the pedal feel was back and felt ok. But after that first week the odd response has developed again. No leaks anywhere either.

Any suggetions on what to look for or what the culprit might be? I suspect it's a bad seal on piston in the master cylinder at this point, no idea what kind of service job that will be either.

Thanks!

tracer bullet · 08-06-2023, 05:11 PM

Sounds like you're on the right track. The slow, sinking thing usually is the seal on the piston like you mentioned. Odd that the bleeding helped though... there is a chance that air is coming in, which means fluid is coming out, and you just haven't found out where yet. You could consider getting the car on ramps or a hoist and taking off the belly pan to see if it's collecting somewhere you didn't realize up to now.

08-06-2023, 02:12 PM	#1
Scottaw Enlisted Member 12 Rep 36 Posts Drives: 2013 128i convertible Join Date: Oct 2013 Location: Upstate NY iTrader: (0)	Brake Issue - Need help to diagnose 2013 128i with 64K miles, have had it for four years now. A few weeks ago I noticed the brake pedal lost some of it's feel, usually the brakes have been very good, strong and responsive. I replaced rotors/pads at 56K when I got it. The brake pedal now feels almost as if there is leakage across the master cylinder piston. The pedal will slowly sink with a constant pressure on it. I can release the pedal and the bite is immediately at top of the pedal again, but will begin to sink after a few seconds of constant or increased applied pedal force. I had received a promotional service coupon from my local dealer so i had them bleed the brakes, since that was supposed to be part of the 60,000 mile service. The first week or so after driving it the pedal feel was back and felt ok. But after that first week the odd response has developed again. No leaks anywhere either. Any suggetions on what to look for or what the culprit might be? I suspect it's a bad seal on piston in the master cylinder at this point, no idea what kind of service job that will be either. Thanks!
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08-06-2023, 05:11 PM	#2
tracer bullet Brigadier General 2412 Rep 3,548 Posts Drives: '11 135i , '15 X3 35i Join Date: May 2011 Location: Saint Paul, MN iTrader: (1)	Sounds like you're on the right track. The slow, sinking thing usually is the seal on the piston like you mentioned. Odd that the bleeding helped though... there is a chance that air is coming in, which means fluid is coming out, and you just haven't found out where yet. You could consider getting the car on ramps or a hoist and taking off the belly pan to see if it's collecting somewhere you didn't realize up to now.
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