Device for discontinuing and automatically restoring the operational function of a spring brake actuator

A spring brake actuator for a brake system having a spring holding a braking piston in a braking position, comprising in combination, a cylinder, a piston rod movable to actuate said braking piston in braking and release directions, a fluid responsive piston movable in said cylinder in one direction for overcoming said spring by fluid pressure to release the braking piston, said spring actuating the piston in an opposite direction for braking at falling of said fluid pressure, a non-rotatable threaded member movable with the piston rod, means including a first rotatable member coaxial with the piston coupling the piston rod to the piston for movement by said piston, a second rotatable member coaxial with the piston rod connected for simultaneous rotation with said first member, said first rotatable member in non-self locking thread engagement with said non-rotatable member, manually releasable lock means locking to the cylinder the second one of said rotatable members in its direction of rotation by said thread engagement caused by the spring when actuating the braking piston in the braking direction wherein the second rotatable member is connected to the first rotatable member in a way only allowing relative axial movements therebetween and means holding one of said rotatable members axially immovably arranged in the cylinder, said lock means comprising a plunger mounted in the cylinder and spring-biassed means holding the plunger into engagement with the second rotatable member.