Abstract: A disc brake primarily for use in a power transmission train between a drive motor and a gear speed reducer features a standardized compact and simplified hydraulic module containing a single piston and a coacting floating cylinder head and an adjusting collar to regulate the movement of the piston. The standardized module is compatible with and operationally connectable to a variety of brake disc assemblies of differing size and torque arresting capacity. The disc brake is biased "on" by constantly acting spring means behind the piston in the hydraulic module when the hydraulic system is depressurized. The brake also possesses a fail-safe capability and has a very desirable "soft feel" during operation due to hydraulic cushioning between the piston and floating cylinder head.