A piston pump for use in a homogenising apparatus, comprising: - a membrane means separating a product side for a fluid product from a hydraulic side for a hydraulic fluid, said membrane means comprising a single layer or two layers mutually spaced in such a way as to define an intermediate chamber comprising a service fluid; - a reciprocating piston operatively active on the hydraulic fluid; - valve means configured to establish a selective fluid communication between the hydraulic side and a tank containing the hydraulic fluid, wherein said valve means comprise: - a first valve arrangement (5, 6) that, in a working condition of said piston pump, is configured to discharge the hydraulic fluid having a pressure over a first predefined threshold from the hydraulic side towards the tank and to draw the hydraulic fluid from the tank to the hydraulic side in response to a pressure in the hydraulic side dropping below a second predefined threshold, said first valve arrangement (5, 6) comprising a first overpressure valve and an anti-cavitation valve, characterised in that said valve means further comprise: - a second valve arrangement (7, 8) that, in at least one non-working condition of said piston pump, is configured to discharge the hydraulic fluid having a pressure over a third predefined threshold from the hydraulic side towards the tank, wherein the second valve arrangement (7, 8) comprises a second overpressure valve that, in the at least one non-working condition of the piston pump, is configurable in: - an open position in response to the hydraulic fluid having a pressure over the third predefined threshold so as to discharge the hydraulic fluid from the hydraulic side towards the tank; - a closed position in response to the hydraulic fluid having a pressure under the third predefined threshold, the second valve arrangement (7, 8) comprising also a check valve that, in the working condition of said piston pump, is configured to prevent a flow of the hydraulic fluid from the hydraulic side towards the tank, the second overpressure valve being interposed between the tank and said check valve. - a membrane means separating a product side for a fluid product from a hydraulic side for a hydraulic fluid, said membrane means comprising a single layer or two layers mutually spaced in such a way as to define an intermediate chamber comprising a service fluid; - a reciprocating piston operatively active on the hydraulic fluid; - valve means configured to establish a selective fluid communication between the hydraulic side and a tank containing the hydraulic fluid, wherein said valve means comprise: - a first valve arrangement (5, 6) that, in a working condition of said piston pump, is configured to discharge the hydraulic fluid having a pressure over a first predefined threshold from the hydraulic side towards the tank and to draw the hydraulic fluid from the tank to the hydraulic side in response to a pressure in the hydraulic side dropping below a second predefined threshold, said first valve arrangement (5, 6) comprising a first overpressure valve and an anti-cavitation valve, - a first valve arrangement (5, 6) that, in a working condition of said piston pump, is configured to discharge the hydraulic fluid having a pressure over a first predefined threshold from the hydraulic side towards the tank and to draw the hydraulic fluid from the tank to the hydraulic side in response to a pressure in the hydraulic side dropping below a second predefined threshold, said first valve arrangement (5, 6) comprising a first overpressure valve and an anti-cavitation valve, characterised in that said valve means further comprise: - a second valve arrangement (7, 8) that, in at least one non-working condition of said piston pump, is configured to discharge the hydraulic fluid having a pressure over a third predefined threshold from the hydraulic side towards the tank, - a second valve arrangement (7, 8) that, in at least one non-working condition of said piston pump, is configured to discharge the hydraulic fluid having a pressure over a third predefined threshold from the hydraulic side towards the tank, wherein the second valve arrangement (7, 8) comprises a second overpressure valve that, in the at least one non-working condition of the piston pump, is configurable in: - an open position in response to the hydraulic fluid having a pressure over the third predefined threshold so as to discharge the hydraulic fluid from the hydraulic side towards the tank; - a closed position in response to the hydraulic fluid having a pressure under the third predefined threshold, - an open position in response to the hydraulic fluid having a pressure over the third predefined threshold so as to discharge the hydraulic fluid from the hydraulic side towards the tank; - a closed position in response to the hydraulic fluid having a pressure under the third predefined threshold, the second valve arrangement (7, 8) comprising also a check valve that, in the working condition of said piston pump, is configured to prevent a flow of the hydraulic fluid from the hydraulic side towards the tank, the second overpressure valve being interposed between the tank and said check valve.