...... a) I asked and was told they were post mount and b) the calipers are using an 70mm spacing that looks like post mount but uses M5 bolts like flat mount. ......
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So, now I’m wondering what calipers might be compatible with the LTWOO levers. ......
What I want to say is that Ltwoo does not adhere to any known standards, whether it's flat mount or post mount. Ltwoo has designed its own specification and provides a bracket with the kit to install it on most flat mount frames. Therefore, you will find that the included screws are M5 size, not M6. Additionally, this Ltwoo-specific specification was not initially designed to accommodate post mount frames, meaning it cannot be installed on any post mount frame.
As for the second question, it is actually easily solvable. Any Shimano hydraulic disc brake caliper compatible with mineral oil systems can be used with Ltwoo's control lever, including but not limited to models like MT200, M6100, R7170, GRX400, and so on.
You just need to ensure that the syringe, olive head, and hose are compatible. I recommend using BH90 components across the board because BH90 hoses can be directly installed on all BH59 compatible components. As long as you ensure proper hose installation, you don't have to worry about any compatibility issues.
I have paired my GR9 control lever with the UR300 flat mount caliper using BH90 hose, syringe, and olive head, and the braking performance and feel are excellent. I have been using it smoothly for nearly 5,000km without any issues of brake fluid leakage or loss of braking power. So, feel free to boldly pair them together.