Shimano's Zee BR-M640 Hydraulic Disc Brake Caliper with Pads features a four-piston caliper and is compatible with Saint and Zee levers. Shimano uses ceramic pistons to insulate the mineral oil from braking heat, helping to keep engagement from fading during long periods of heavy use. The H03C metal, or sintered, pads are ideal for heavier rider/bike combinations and braking in wet conditions. They pair well with the ceramic pistons since metal pads tend to absorb heat into the calipers rather than repelling it into the rotor, so the pistons help serve as a buffer between the pads, the calipers, and the oil. Metallic pads also lack the initial engagement bite of resin, but--like a big-hit gravity machine compared to the light suspension of an XC ride--metal pads will still be grabbing even while they're superheated at the end of a gnarly run. The D01S resin pads are better at repelling heat into the rotor, which protects the oil and keeps braking consistent. They also bite more readily than metallic pads, and they reduce both the break-in time and engagement squeal. The trade-off to faster bite, less noise, and cooler oil is that the pads themselves tend to fade at high heat, so they're best for rides that require less frequent, more modulated braking. Shimano's Zee BR-M640 Hydraulic Disc Brake Caliper is compatible with post-mount adapters, available separately, for rotors sized from 160, 180, and 203mm.