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I have a pair of Shimano Deore BR-M615s on my mountain bike with about 3,000 miles and 5 years on them. Lately I've noticed that both brakes have too long of "pull" before providing braking. That is, the distance from when I start to pull the lever to when the pads engage feels too long, perhaps 3/4s of an inch or so. The braking power after the pads are applied seems fine. I think the pads and rotors are within spec.

When I remove the wheel and pads, "exercise" the pistons, and clean/lube with mineral oil, the pistons are quite finicky. Sometimes only one moves, other times after some the fidgeting, only the other piston will move. One piston does not return all the way back into the caliper (perhaps 1/2 millimeter protrudes). Sometimes the pistons are cock-eyed, extending from the caliper at an angle. Sometimes I notice air bubbles in the lube oil between the pistons and the caliper. It's difficult to get both pistons extending evenly from the caliper. After some fidgeting, I can improve the "pull" distance, but as the pads wear I have to fidget more.

I am considering replacing the calipers with the BR-M6000, but wanted some input before pulling the trigger. What makes the most sense to try next? Overhaul the current brakes? New calipers? New levers, hoses, and calipers? Live with the fidgeting?

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    The measurement you're talking about is known as "free stroke". Has it always been bad? Is it possible that you just didn't care/notice until now? The uneven piston retraction is normal and not a big problem unless a piston is legitimately frozen in place. – MaplePanda Jul 27 at 5:37
  • Have you ever changed the brake fluid? 3000 miles is quite short, but depends on riding conditions and whether you put the bike away wet/dirty. Brake fluid can be hydroscopic, absorbing water, which generally messes with the compression part. If the fluid is old, it may have sucked in water past the seals, which can also contribute to corrosion and a non-smooth more for the pistons to ride in. – Criggie Jul 27 at 11:16
  • @MaplePanda The "free stroke" has varied over the years, but I feel like it's getting worse lately. In particular, I can fidget to get the free stroke to an acceptable amount, but as the pads wear, the pistons don't seem to "take up the slack" and move further from the caliper and closer to the rotor. – frito_mosquito Jul 27 at 16:09
  • @Criggie The shop says they did a full bleed recently, but perhaps they did not replace all of the fluid. – frito_mosquito Jul 27 at 16:11
  • @frito_mosquito that's good - a full bleed would replace all the fluid. – Criggie Jul 27 at 19:27
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What you describe in the first part of your question is normal. Longer lever travel may indicate you need to bleed the brakes. Uneven caliper piston movement is caused by contamination of the piston seals with dirt. Park Tool has a video about cleaning pistons and seals.

However this is bit more worrying:

Sometimes the pistons are cock-eyed, extending from the caliper at an angle. Sometimes I notice air bubbles in the lube oil between the pistons and the caliper.

The pistons should not be able to extend at an angle. That's not 'lube oil' between the pistons and caliper, it's the hydraulic fluid (mineral oil in Shimano's case) and you should not be seeing any of it getting past the seals (or leaking anywhere).

This makes me suspect that the seals are worn out. The Park Tool video I linked above shows how they work.

Seems to me that it should be possible to replace the seals in hydraulic brake calipers, but I've never heard of anyone doing it - someone will maybe add a comment about that.

I would not assume that calipers and levers from different groupsets are compatible. I tried looking this up in Shimano's compatibility information but inter-groupset caliper and lever compatibility is not listed for MTB groupsets.

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    When I said I was "exercising" the pistons, I was performing exactly what is done in your linked Park Tool video. The trouble is that even after performing the cleaning, the pistons still do not extend evenly. The "lube" I was referencing was the mineral oil I had put on the outside of the pistons, and the "bubbles" were in that external oil that I had introduced, not leaking from inside the caliper. – frito_mosquito Jul 27 at 16:06
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    @frito_mosquito You should never put any kind of lube on brake calipers. This risks contaminating the pads The pistons do not need any kind of lubrication, the hydraulic fluid deals with that internally. Bubbling of the lube you applied suggests you have air in the system and further suggests the seals are not working properly. The extra lube may in fact have affected the seal material. What I would do is clean the pistons as per Park Tool, and get the brakes bled. – Argenti Apparatus Jul 27 at 16:14
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    Sorry if I have not been clear. I have not applied any kind of lube to the pistons to the brake calipers. I applied mineral oil as per the video. I have already cleaned the pistons as per the video. – frito_mosquito Jul 27 at 16:39
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    There are no replacement seals available, likely to avoid people screwing up a brake rebuild and dying after a brake failure. All calipers and levers should be cross compatible, as Shimano uses the same diameter pistons throughout the entire lineup. – MaplePanda Jul 27 at 19:28

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