A locking assembly for preventing the accidental let out of a slack adjuster is provided. The elongated rod member of the slack adjuster has a first portion positioned within at least a portion of the slack adjuster's elongated hollow housing member and a second portion extending out of an end of this elongated hollow housing. A jaw member may be provided which is engageable with this second portion of the elongated rod member at a location remote from the end of the elongated hollow housing member. The locking assembly comprises an end cap engageable with the end of the elongated hollow housing. This end cap includes a portion extending substantially parallel with the second portion of the elongated rod member. At least one aperture is formed through this parallely extending portion of the end cap. A member is provided which is engageable within this at least one aperture and with the second portion of the elongated rod member or the jaw member for preventing longitudinal movement of the elongated rod member with respect to the elongated hollow housing member and consequently for preventing the accidental let out of the slack adjuster.