The present invention provides an attachment (1) for a fluid-operated percussion drill tool (2), the drill tool having a backhead assembly (4) for attachment to a drill rod (10). The attachment comprises a shaft (3) attachable to the backhead assembly, wherein the shaft comprises a central bore (16) in fluid communication with a pressure bore of the drill rod. The attachment further comprises a sleeve (5) co-axially slidably mounted on the shaft, the sleeve comprising an outer shoulder (7) at a rear end thereof, and at least one rearwardly-directed exhaust passage. Pressurised air is supplied from the central bore of the shaft to urge the sleeve rearwards into a closed position in which air is prevented from exhausting through the exhaust passage. When a restriction is encountered at the shoulder, the sleeve moves forward to an open position in which air is permitted to exhaust through the exhaust passage adjacent the restriction.