The handbrake function with rear discs seems to challenge MMC starting with the Aero's then ARP4. There are various solutions employing separate handbrake caliper, drum within disc rotor, dedicated additional master cylinder to hydraulics etc. Having said that our F54 Mini has one of the best handbrakes I've ever encountered using a single piston floating caliper incorporating both hydraulic and cable actuation. The latter with electric assistance though - maybe that's the answer
