A manual disconnecting device is provided for disengaging the brake push rod in a spring brake actuator having a movable piston holding the spring compressed by fluid pressure during normal operation. The device once manually disconnected is automatically reconnected upon a normal spring compression stroke of the piston. The disconnecting device provides a clutching device between the piston and the push rod engaging a nut threaded on the push rod with non-self-locking threads. In one embodiment the nut us journalled in the piston. In another embodiment there are clutch surfaces on both the piston and nut which are disengaged by a rotary motion from a special tool.