Issue with Shimano Hydraulic Disk Brake
03-20-23, 06:53 AM
#1
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Issue with Shimano Hydraulic Disk Brake
So, I bought and installed a new pair of Shimano BL-M4100/BR-MT420 brakes with J-kit. Given that the hydraulic line to the rear caliper is internally routed through both halves of the folding frame, I expected it to be the one that might give me issues; instead, it's the front brake that seems to be malfunctioning.
While the rear brake lever has a good amount of free stroke and a nice, solid, and distinct bite point, the front brake lever barely has any free stroke at all, and engagement is spongy and vague. I tried to do a lever bleed, which only helped a little, but it's far from ideal.
Looking down at the caliper, I can see that the pads are sitting too close to the rotor and all four pistons seem to be sticking out too far - and somewhat unevenly - making it impossible for me to center the caliper around the rotor. The wheel does spin freely, but not without the rotor rubbing. I tried to reset the pistons back into the caliper bores, but they would stick back out as soon as I pull the lever exactly twice.
What can I do at this point? Do you think a full bleed would help? Would I need to take the caliper apart and reset each piston?
While the rear brake lever has a good amount of free stroke and a nice, solid, and distinct bite point, the front brake lever barely has any free stroke at all, and engagement is spongy and vague. I tried to do a lever bleed, which only helped a little, but it's far from ideal.
Looking down at the caliper, I can see that the pads are sitting too close to the rotor and all four pistons seem to be sticking out too far - and somewhat unevenly - making it impossible for me to center the caliper around the rotor. The wheel does spin freely, but not without the rotor rubbing. I tried to reset the pistons back into the caliper bores, but they would stick back out as soon as I pull the lever exactly twice.
What can I do at this point? Do you think a full bleed would help? Would I need to take the caliper apart and reset each piston?
03-21-23, 02:20 PM
#3
Senior Member
Did you open the lever bleed port when you pushed the pistons back in? It sounds overfull. If so, yeah, full bleed. Also another less likely possibility (for a new brake) is that the pistons are sticking and aren't contracting properly. Lubricating the pistons with a little mineral oil before pushing them back in isn't a bad idea.
03-22-23, 01:48 AM
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Did you open the lever bleed port when you pushed the pistons back in? It sounds overfull. If so, yeah, full bleed. Also another less likely possibility (for a new brake) is that the pistons are sticking and aren't contracting properly. Lubricating the pistons with a little mineral oil before pushing them back in isn't a bad idea.
I then did a full bleed (syringe at caliper; funnel at lever,) making sure that clean, bubble-free oil came out at both ends. Bite point feel is slightly better now, but nowhere near good and positive as the rear brake (or the MT401/MT420 on my other bike) and there's still very, very little free stroke.
At this point, I'm thinking of contacting the sellers as a next step and see what possible course of action there is.
