As far as I know there's nothing stopping you from using Shimano Post-mount M-series calipers with road hydraulic brake levers. Pretty sure first gen hydro brifters often came connected to BR-M785 (XT) calipers. PM caliper to IS frame/fork is a common adapter.
As far as I know there's nothing stopping you from using Shimano Post-mount M-series calipers with road hydraulic brake levers. Pretty sure first gen hydro brifters often came connected to BR-M785 (XT) calipers. PM caliper to IS frame/fork is a common adapter.