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M2 Technical Topics > Suspension | Brakes | Chassis > Brake piston stuck in open position

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      11-17-2018, 01:09 PM   #1
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Unhappy Brake piston stuck in open position

Hy guys,

Today I started swapping my brake pads. Certainly not the first time I do that on the M2... but unfortunately this time didn't work well

- I removed the pads clip from the front brake caliper
- I spreaded the old pads from the rotors relatively easily (with the hands)
- I removed the 2 pads
- I spread further the 2 pistons of the 1st pad and inserted the new pad
- I spread the 1st piston of the other pad without any issue of resistance
- ... and then I tried to spread the 2nd piston of that same 2nd pad... but it was completely stuck !

I tried several times to unblock it but it was impossible. I also started the engine in case an ABS system would help to release it but it was not the case. I even braked slightly but nothing to do. Now the 2 pistons of that pad are totally stuck. There is some space between the rotor and the 1st piston but it does not move. There is less than 0.01 mm space betwen the 2nd piston and the rotor (thus too narrow to insert a tool) and it is also impossible to move it.

I have searched several forums and DIY videos but all of them address issues (of old cars) with piston seized in closed position, which is not my case

=> Do you have any suggestion to fix this ?

If possible I'd rather avoid opening the purge screw .

Is it possible that the piston was pushed too far from the calliper (for instance when I braked slightly) ? In such case is there a specific process to put it back in position (not easy as it is almost glued on the rotor) ?

Thanks for your kind assistance on this !
Cheers,
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      11-17-2018, 04:11 PM   #2
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I'm assuming u pressed the peddle while there was no pad and now the piston is nearly touching the rotor.

Would be hard to tell but is this piston sitting on a slight angle due to the amount it has come out as this would make it hard to push back in. If you have enough room to clear the pistons from the disc you could remove the caliper as this will give u more space to work with otherwise opening the bleed screw may release some pressure to reseat correctly.

The other thing you may be able to do is unscrew the rotor if there is a lock screw and use the rotor as leverage ie pull the rotor to the outside of the caliper to see if u can get the piston to seat. This will only help if it is the outside pistons though
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      11-19-2018, 07:56 AM   #3
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Do you track your car?
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      11-19-2018, 01:10 PM   #4
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Quote:
Originally Posted by ZM2 View Post
Do you track your car?
OFF COURSE !!!

Don't you ?
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      11-19-2018, 01:14 PM   #5
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Finally I fixed it

It was almost freezing when I worked and the brake oil seems much less fluid at low temp, that's probably what explained the issue (worsened when I slightly push the brake paddle).

I completely removed the full calisper, which gave more manoeuver room to use an appropriate tool and gently push the pistons successfully.

Lessons learneds:
- try to always keep the other brake pad when you replace one
- more difficult to work when it is really cold
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      11-19-2018, 01:26 PM   #6
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I had to trash a set of OEM calipers from heavy tracking bc the dust boots melted and a piston started sticking. If you have the issue again, keep this in mind.

Also, be very mindful of your pad wear on your next outing. If that piston sticks, the other will still press out and you’ll get uneven wear. I also killed one pad in a set of track pads in one day bc of the uneven wear.
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      11-19-2018, 01:44 PM   #7
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      11-19-2018, 03:18 PM   #8
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Quote:
Originally Posted by ZM2 View Post
I had to trash a set of OEM calipers from heavy tracking bc the dust boots melted and a piston started sticking. If you have the issue again, keep this in mind.

Also, be very mindful of your pad wear on your next outing. If that piston sticks, the other will still press out and you’ll get uneven wear. I also killed one pad in a set of track pads in one day bc of the uneven wear.
Also there is a piston replacement/upgrade kit on the market.
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      11-19-2018, 04:57 PM   #9
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Quote:
Originally Posted by Tano View Post
Also there is a piston replacement/upgrade kit on the market.
Yeah, but by the time I rebuilt and repainted the caliper, I could buy someone else’s take off’s cheaper, or at least easier.

I’m running an AP BBK now, so will worry about the stock brakes if I ever sell the car.
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