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?

Sounds like trapped air at the caliper. Do a full bleed.

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.

Yup, did all this. I first tried pushing the pistons out further, lubricating them with mineral oil, and pushing them back in (while leaving the lever bleed port open) to make sure that the seals are reset in the fully retracted position. Not much use there.

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.