Braking unit for vehicle, of the type comprising an axial actuating helical spring maintained in release position, against its elastic reaction force, through a screw-nut system rotatively driven by an electric motor, the spring acting upon the brake through the intermediary of a hydraulic master cylinder of which the piston is axially connected to an abutment ring of the spring also mechanically connected, via a screw-nut system to the rotor of an electric motor, such an unit being applied to a spring braking system for a vehicle possibly with a built-in park brake.