Abstract: A disc brake caliper assembly for a rotary disc brake of a vehicle which has a wear adjustment mechanism is provided. The disc brake caliper assembly (10) includes a caliper housing (14) with a piston (28) slidably mounted therein, but prevented from rotating by a lug (30) on a brake shoe (16) secured thereto which keys in a groove (32) in the piston. An actuator screw (44) and actuator nut (66) are positioned in an internal bore (65) in the piston for mechanical actuation of the brakes. If the brake pads (20,22) wear, on hydraulic actuation of the brakes, a thrust washer (70) engages and moves the actuator nut axially along the actuator screw to a new position which restricts reverse movement of the piston to a new (wear adjusted) position relative to the caliper housing. An actuator cone (68) having a friction clutch surface biased into engagement with a friction clutch surface on the piston prevents reverse movement of the actuator nut.