A valve actuation mechanism for an internal combustion engine includes rockers moved by a camshaft, each rocker being adapted to exert a valve opening force on at least a portion of a valve actuation mechanism of each cylinder belonging to a housing of the engine, via an engine brake activation piston, housed in a bore of the rocker, and movable with respect to the rocker under action of a fluid pressure raise in a chamber fluidly linked to the bore, from a first position, in which an engine brake function is deactivated, to a second position, in which a roller of the rocker reads at least one engine brake bump of a cam of the camshaft so as to perform the engine brake function. Each rocker includes a discharge valve movable in translation with respect to the rocker, adapted to reduce fluid pressure in the chamber. The engine brake system includes, for each rocker, a stopper fast with the housing and adapted to exert, on a portion of the piston, a force for opening the discharge valve when the activation piston has to be moved from its second position to its first position.