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Assume the smaller syringe allowed for higher pressures?
Yep
Did you bleed it lever to caliper to push it out rather than into the system?
With shimano; I pushed from lever to caliper into the funnel. I think the 3rd party (like Epic bleed? Or aliexpress ones) bleed kits have syrenges that work on the funnel end of caliper so you bleed it like sram, I think its a better approach. Slightly more complicated.
Did you not follow my MT400 issue, it was identical .. I had a mini oil explosion and something stuck was removed with black oil in funnel.
Try a different/smaller syringe, that worked for me.