I want to use the XT 8120 4-piston caliper to my GRX 815 STIs to increase my braking power. After researching, I still haven't found a definite answer. I've also looked into the Hope RX4+ 4-piston calipers, however there are some users complaining with difficulty in fully bleeding the calipers due to the design.

Any one out there tried running STIs with 4-pot calipers?

  • I run first-gen RX-4 and the bleed is pretty much the same as other MTB/road calipers.
    – Klaster_1
    Oct 10 at 10:41

Hm. Let’s think about this for a second. Info is specific to Shimano:

  • Flat mount MTB calipers are just relabeled road calipers. Therefore, MTB levers can properly actuate road calipers.
  • Accordingly, since road levers and MTB levers can both actuate a 2-piston caliper, one can then deduce that road levers can properly actuate MTB 2-piston calipers.
  • Since 2-piston and 4-piston MTB calipers use the same levers, one can therefore conclude than road levers can actuate 4-piston MTB calipers.

Of course, the logic is funky at best, but at least there’s no glaring reason for incompatibility.

  • The exception is Saint/Zee levers, which I believe use a smaller piston in the lever to generate more power, but that is irrelevant.

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