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pascalo
BrickMan
Shimano road leaky piston syndrome!
Shop might not have done a good job of cleaning up though after a bleed, so always that. Someone else already said it, bleed block or tyre lever and clamp it overnight with a zip tie/pedal clip strap.
They are renowned for splitting pistons, USUAL non shimano faults are....
Customer rammed piston back in on a jaunty angle = cracked piston
customer chipped edge of piston with rotor when putting wheel back in = cracked piston
customer got hungry and decided to attack pistons with teeth/hammers/claws and now = cracked pistons.
Etc
Sometimes they crack at the edge, sometimes across the whole thing, and quite a few times they crack down the centre, creating an inner and an outer piston! The inner piece gets stuck against back of bore, the outer piece leaks fluid and crumbles away. Mean time the brakes sort of work.
You can buy aftermarket metal or phenlolic plastic pistons and just stick them in, very little else wrong to go in the caliper, you don't get the same corrosion issues inside the reccess that the o-ring sits in like Sram/Avid/Hope/Formula brakes due to been sat in minging 10 year old dot fluid