An interlock for a slider incorporates a cover that moves between a covered position and an uncovered position. In the covered position, the cover prevents access and movement of a handle for actuating locking pins on the slider. The cover may be pivoted to an uncovered position at which the handle can be actuated. Generally, the handle is pulled away from a trailer frame for the slider to move the locking pins out of holes in the trailer frame. When the cover has moved away from the covered position, the cover actuates a valve member that sets at least one park brake. To release the park brakes, the cover must be moved back to the covered position. The cover cannot be moved back to the covered position until the handle is in a position indicative of the locking pins being fully received in the holes in the trailer frame. In this manner, the cover ensures that the locking pins are fully engaged in the trailer frame, and further provides a visual feedback to an operator of why the park brakes may not be released in the event the locking pins are not fully engaged.