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I've replaced the brake pads of my bike, Shimano BR-M395 with some BS03S (there were originally BS01S, but I have been told they are identical), but the new pads just keep on rubbing.

I've even taken the pads off myself and 'reset' the pistons, by levering both of the pistons as far away from the middle as possible. There is plenty of space, but as soon as the brake levers are engaged the pads return to hugging the disc. There just doesn't seem to been any mechanism to cause the pads to separate (yes the spring clip between the pads is fitted!).

I've performed no maintenance on the hydraulic system; do I need to bleed the system or replace the oil? If so can someone explain how to do this, or do I have to take it to a shop?

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B03S is indeed a successor to B01S and is interchangeable with it.

What you're describing is a brake that isn't retracting its pistons correctly.

Air contamination isn't a direct cause of this problem and bleeding itself isn't a solution. However, this problem often occurs on Shimano brakes that have been heavily used with the pistons running very extended (potentially overextended) for a long time. If that's true, it's common for the fluid that comes out of such brakes to be very dirty and good to replace in its own right.

To attempt fixing it in a way that's guaranteed not to hurt it, you do need Shimano mineral oil. Other brake mineral oils sold as generic service replacements (Finish Line, Muc-off, etc) would also be okay.

Take the pads back out and using the brake lever, intentionally extend the pistons such that you can see about as much as you ever would in use. Thoroughly clean the entire brake with something appropriate that won't attack the seals, so either alcohol or a soap/detergent sanctioned for the purpose. Once it's clean, use a cotton swab to slather the exposed pistons with oil, then push them back in. Extend them out again and repeat the process. You're looking to reduce the feeling of friction as you push them in. Once you've done that until you see no further improvement, you could bleed it at that juncture if you wanted to know you were "doing everything." Then clean the caliper again to get all the oil off and leave it with zero residue, then set the brake up again.

Those steps are about as far as you can go with a Shimano brake, and if it doesn't work then the next plan is replace. Aftermarket repair pistons do exist that purport to be able to fix such problems, but a whole new caliper is a better option at that point until you're talking about the more expensive brakes.

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    Worth noting at any point the above steps, the 'choosing to live with it' option is always open.
    – mattnz
    Nov 7, 2022 at 20:49
  • Hi - thanks for the advice - this is exactly what I needed. Extending the piston and cleaning there solved the issues. When I originally cleaned I just focussed on removing the dirt I could see, with the piston extended I kept on running an oiled rag around the piston until it was clean.
    – DrZaphod
    Nov 20, 2022 at 22:29
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I'm happy to see you get away with cleaning the pistons...

Sometimes, you can't - that is said, "lazy pistons" in petrolhead speak, occurs to car brakes. There are rectangular seals inside calipers that retract pistons. They tend to go bad after >10 years on cars, which happened to me in Deore brakes.

I saw replacement o-rings at AliExpress. Next time, you may need them. Just a reminder.

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