The FS Series of spring applied brakes require pressure (normally hydraulic) for disc release. Braking force is provided by a stack (or stacks) of Belleville spring washers, capable of storing enormous force. When presurized the force moves the piston(s) to compress the spring washer stack(s), releasing the disc. The FS20 Series features replaceable, high-grade friction material, aluminum die-cast construction and Zinc plated grade 8 bolts. Buna-N seals are standard. Maximum hydraulic pressure rating is 1500 PSI Non Shock. Minimum hydraulic pressure to release brake is 750 PSI.