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I changed the rear brake pads on my BMW last week, following the steps outlined in this video.

Unfortunately the brake pedal now feels spongy. The pedal feel was firm prior to the brake swap, so it is definitely a sudden change and not a gradual degradation of brake fluid.

One thing I forgot to do was undo the brake fluid reservoir cap before pushing the piston calipers back with a C-clamp, so I suspect that the the sponginess is due to trapped air in the system since the reservoir was never exposed to outside air.

Or is it possible that the extra pressurization of the system via the C-clamp resulted in moisture absorption? The only issue I see with this reasoning is that the system was never exposed to outside air.

  • Does the pedal feel spongy when driving the car or does the pedal feel spongy with the car off after pressing the pedal at least 5 or six times? – vini_i Sep 20 '15 at 15:59
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    In the video, he should have had the rotor turned or put a new rotor on it ... I wouldn't have used it the way it was, that's for sure. The reservoir cap on/off wouldn't have caused an issue. C-clamp would not cause moisture absorption. Best bet is to flush/bleed the entire system. – Pᴀᴜʟsᴛᴇʀ2 Sep 20 '15 at 19:37
  • Did your "Brake Service in XXX miles" indicator light come on? – mikes Sep 21 '15 at 9:41
  • @vini_i : It feels spongy with the car off. It feels spongier with the car on (I assume because of the vacuum assist) – Zaid Sep 21 '15 at 9:56
  • @mikes : This is a '99 M5. No such indicator – Zaid Sep 21 '15 at 9:58
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Two things to look for.

1) air in the brake fluid.

2) incorrectly assembled brake pads, especially the anti noise shims.

Air in the brake fluid is the most common cause of low, spongy brake pedal feel. Moisture in the fluid will not substantially change the pedal feel until the fluid temperature exceeds the boiling point of any water in the fluid, then the pedal feel gets very soft. Brake fluid absorbs water if exposed, so the systems are to remain sealed. Brake fluid older than three years or any color other than that of tea should be flushed.

Improper assembly of the brakes can cause the pistons to be pushed back too far into their bores when the brakes are released. this cause a longer than normal stroke of the brake pedal before resistance is felt.

Also new brake pads installed without surfacing the rotors can result in more pedal effort required for normal braking.

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As stated prior, if the rotors weren't machined or lightly surfaced when the pads were changed, that will give you a spongy feel as you are not stopping as effectively and takes more effort to slow down.

With new rotors and pads, the machine marks on the rotors help to break in the rotors and pads together. Once broken in, they will work well as they wear together.

If my rotors have no runout (warping) when I change pads, I will use an abrasive disk on the rotor to scuff and break the shine on both sides. Do not take any material off. The rough surface will help mate and break in the pads. After changing them, I take the car out on a short run and run it hard, then stop fully hard at least a half dozen or more times. This helps the break in... while you may initially smell the pads when heated, that will go away.

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If the brakes are always spongy, it is air bubbles.

If the brakes are only spongy when hot, it is moisture.

0

Pedal feel is always softer until the pads have bedded into the rotors properly.

Make sure that you follow a proper bed-in procedure before deciding that you have a problem.

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