A pressurized fluid down hole hammer drill comprising: a drill housing; a drill bit; a piston arranged for reciprocating movement in the drill housing and to impact on the drill bit; a chamber defined between the drill bit and the piston; a supply port for receiving pressurised fluid from an external supply; a piston conduit for conveying fluid from the supply port to the chamber to drive the piston; and a port tube for at least conveying fluid from the chamber to an exhaust, characterised in the hammer drill further comprising: a spring-biased check valve moveable within an internal bore of the port tube; a seal tube in the form of a substantially cylindrical tube; and at least one exhaust port defined at least in part by the drill bit for controlling the flow of fluid from the chamber to the port tube wherein, in use: the seal tube moves relative to both the piston and the drill bit; at least a part of the seal tube moves within at least a part of the piston and co-operates with the movement of the piston to close the exhaust port when the piston is moving in a first direction and is at a first position and to open the exhaust port when the piston is moving in a second direction opposite to the first direction and is at a second position different to the first position; at least a part of the seal tube moves within at least a part of the drill bit at least when the exhaust port is closed; and wherein the spring-biased check valve is arranged to release pressurized fluid from the chamber for a period after the exhaust port has closed, but prior to the piston striking the drill bit. a drill housing; a drill bit; a piston arranged for reciprocating movement in the drill housing and to impact on the drill bit; a chamber defined between the drill bit and the piston; a supply port for receiving pressurised fluid from an external supply; a piston conduit for conveying fluid from the supply port to the chamber to drive the piston; and a port tube for at least conveying fluid from the chamber to an exhaust, characterised in the hammer drill further comprising: a spring-biased check valve moveable within an internal bore of the port tube; a seal tube in the form of a substantially cylindrical tube; and at least one exhaust port defined at least in part by the drill bit for controlling the flow of fluid from the chamber to the port tube wherein, in use: the seal tube moves relative to both the piston and the drill bit; at least a part of the seal tube moves within at least a part of the piston and co-operates with the movement of the piston to close the exhaust port when the piston is moving in a first direction and is at a first position and to open the exhaust port when the piston is moving in a second direction opposite to the first direction and is at a second position different to the first position; at least a part of the seal tube moves within at least a part of the drill bit at least when the exhaust port is closed; and wherein the spring-biased check valve is arranged to release pressurized fluid from the chamber for a period after the exhaust port has closed, but prior to the piston striking the drill bit. a spring-biased check valve moveable within an internal bore of the port tube; a seal tube in the form of a substantially cylindrical tube; and at least one exhaust port defined at least in part by the drill bit for controlling the flow of fluid from the chamber to the port tube wherein, in use: the seal tube moves relative to both the piston and the drill bit; at least a part of the seal tube moves within at least a part of the piston and co-operates with the movement of the piston to close the exhaust port when the piston is moving in a first direction and is at a first position and to open the exhaust port when the piston is moving in a second direction opposite to the first direction and is at a second position different to the first position; at least a part of the seal tube moves within at least a part of the drill bit at least when the exhaust port is closed; and wherein the spring-biased check valve is arranged to release pressurized fluid from the chamber for a period after the exhaust port has closed, but prior to the piston striking the drill bit.