Hope RX4+ Disc Brake Caliper - Rear Flat Mount Direct +20mm Mineral Oil RedHope RX4+ Disc Brake Caliper Rear Flat Mount Direct +20mm Mineral Oil Red. The Hope RX4+ mineral oil type caliper builds on 25 years of experience designing and manufacturing hydraulic disc brakes and transfers that knowledge into the world of road and cyclocross drop bar brakes. A precision CNC machined monobloc design results in a stiffer caliper to give unrivaled feel and modulation. Monobloc design creates a stiffer caliper reducing the fuzzy