Hydraulic disc brake. E-Bike ready: Yes. Mounting caliper: Postmount. Brake fluid: Mineral oil. Number of pistons: 4. The Shimano caliper BR-MT420 is a 4-piston brake caliper that can be mounted on the front or rear. It uses mineral oil as hydraulic fluid, which is non-corrosive and less hazardous than conventional hydraulic fluids. Thanks to the washerless mounting bracket, it allows for quick installation and setup. The caliper is compatible with Postmount or international standard forks when used with adapters. Additionally, it can be used with 6-bolt or Center Lock brake discs. The One Way Bleeding Caliper System makes it easy to bleed brake systems, and the D03S resin pads provide extra protection against wear.