A hydraulic impact mechanism for use in equipment for the treatment of rock and/or concrete comprising a machine housing with a first cylinder bore, a piston that is mounted such that it can be displaced in the first cylinder bore such that it repeatedly executes a reciprocating motion relative to the machine housing during operation exerting impacts either directly or indirectly onto a tool for the treatment of rock or concrete and is connectable to the equipment, and where the piston includes a drive part that separates a first drive chamber and a second drive chamber formed between the piston and the machine housing, and where the first and second drive chambers (105, 120) are arranged such that they contain a driving medium under pressure during operation, and where furthermore the machine housing includes channels that open out into the first cylinder bore and are arranged to contain driving medium during operation and to, with the aid of the piston during its motion in the first cylinder bore, open and close into at least the second drive chamber such that at least the second drive chamber acquires periodically alternating pressure for the maintenance of the reciprocating piston motion, and that positions, for the openings of the channels are arranged along the first cylinder bore so as to be opened and closed by portions of said reciprocating piston holding the second drive chamber closed for the supply or drainage of driving medium that is present in the second drive chamber along a distance between the opening of a first channel in association with a first turning point of the piston and the opening of a second channel in association with a second turning point for the piston, and that the motion of the piston along said distance takes place during compression or expansion of the volume of the second drive chamber, where the magnitude of the volume of the second drive chamber is adapted to obtain a slow change of pressure along said distance, such that the hydraulic impact mechanism provides a valveless hydraulic impact mechanism, where the said second drive chamber comprises a gas accumulator during operation, characterised in that said gas accumulator comprises a second cylinder bore with an accumulator piston (220, 320) mounted such that said accumulator piston is displaceable in the second cylinder bore, in that said accumulator piston (220, 320) separates the driving medium in the second drive chamber from a gas under pressure contained in a closed compartment of the gas accumulator, in that the volume of said closed compartment varies with the frequency of the impact mechanism during operation as a consequence of the reciprocating motion of the accumulator piston in the second cylinder bore, in that the gas accumulator includes a brake chamber (240, 250, 340) for accelerating the braking of the accumulator piston (220, 320) before the turning point of the accumulator piston, and in that the accumulator piston (220, 320) and the brake chambers (240, 250, 340) are designed such that, when the accumulator piston enters a brake chamber (240, 250, 340), a gap of width less than 0.5 mm arises between them, constituting a gap seal between the brake chamber (240, 250, 340) and the second drive chamber.