A valve train assembly for use with an engine exhaust brake system, the valve train assembly comprising: a hydraulic lash adjuster for adjusting for lash in the valve train assembly, the hydraulic lash adjuster comprising: a first body; a second body mounted for reciprocal sliding movement with respect to the first body; a first biasing means biased to move the second body so as to extend the hydraulic lash adjuster; a chamber between the first and second bodies for receiving hydraulic fluid via a hydraulic fluid input valve in response to the second body moving so as to extend the hydraulic lash adjuster; and: a pressure relief valve that is distinct from the hydraulic fluid input valve and that is operable to release fluid from the chamber to enable the second body to be moved so as to retract the hydraulic lash adjuster; wherein the valve train assembly further comprises an actuator means for opening the pressure relief valve; and wherein the hydraulic lash adjuster is for adjusting lash in the valve train assembly between an exhaust valve and an exhaust cam, mounted on a cam shaft, associated with the exhaust valve, wherein, when the exhaust brake system is activated the exhaust valve is openable by exhaust manifold back pressure and in response to the opening of the exhaust valve the hydraulic lash adjuster is extendible and wherein, the actuator means is arranged so as to open the pressure relief valve to enable the hydraulic lash adjuster to retract to allow the opened exhaust valve to close prior to the exhaust cam causing a lift of the exhaust valve, wherein pressure relief valve comprises a valve head and a second biasing means, characterised in that: the pressure relief valve comprises a valve stem and wherein the pressure relief valve is moveable, in response to a force exerted on the valve stem, from a closed position in which the valve head seals an aperture in the chamber and an open position in which the aperture is un-sealed, wherein the second biasing means biases the pressure relief valve towards the closed position and wherein an end of the valve stem extends outside of the second body. a first body; a second body mounted for reciprocal sliding movement with respect to the first body; a first biasing means biased to move the second body so as to extend the hydraulic lash adjuster; a chamber between the first and second bodies for receiving hydraulic fluid via a hydraulic fluid input valve in response to the second body moving so as to extend the hydraulic lash adjuster; and: a pressure relief valve that is distinct from the hydraulic fluid input valve and that is operable to release fluid from the chamber to enable the second body to be moved so as to retract the hydraulic lash adjuster; wherein the valve train assembly further comprises an actuator means for opening the pressure relief valve; and wherein the hydraulic lash adjuster is for adjusting lash in the valve train assembly between an exhaust valve and an exhaust cam, mounted on a cam shaft, associated with the exhaust valve, wherein, when the exhaust brake system is activated the exhaust valve is openable by exhaust manifold back pressure and in response to the opening of the exhaust valve the hydraulic lash adjuster is extendible and wherein, the actuator means is arranged so as to open the pressure relief valve to enable the hydraulic lash adjuster to retract to allow the opened exhaust valve to close prior to the exhaust cam causing a lift of the exhaust valve, wherein pressure relief valve comprises a valve head and a second biasing means, characterised in that: the pressure relief valve comprises a valve stem and wherein the pressure relief valve is moveable, in response to a force exerted on the valve stem, from a closed position in which the valve head seals an aperture in the chamber and an open position in which the aperture is un-sealed, wherein the second biasing means biases the pressure relief valve towards the closed position and wherein an end of the valve stem extends outside of the second body. a pressure relief valve that is distinct from the hydraulic fluid input valve and that is operable to release fluid from the chamber to enable the second body to be moved so as to retract the hydraulic lash adjuster; wherein the valve train assembly further comprises an actuator means for opening the pressure relief valve; and wherein the hydraulic lash adjuster is for adjusting lash in the valve train assembly between an exhaust valve and an exhaust cam, mounted on a cam shaft, associated with the exhaust valve, wherein, when the exhaust brake system is activated the exhaust valve is openable by exhaust manifold back pressure and in response to the opening of the exhaust valve the hydraulic lash adjuster is extendible and wherein, the actuator means is arranged so as to open the pressure relief valve to enable the hydraulic lash adjuster to retract to allow the opened exhaust valve to close prior to the exhaust cam causing a lift of the exhaust valve, wherein pressure relief valve comprises a valve head and a second biasing means, characterised in that: the pressure relief valve comprises a valve stem and wherein the pressure relief valve is moveable, in response to a force exerted on the valve stem, from a closed position in which the valve head seals an aperture in the chamber and an open position in which the aperture is un-sealed, wherein the second biasing means biases the pressure relief valve towards the closed position and wherein an end of the valve stem extends outside of the second body.