A pressure regulator (5; 6) comprising: - a static downstream body (51; 61) delimiting a low pressure chamber (513; 613) at the outlet of the pressure regulator (5; 6); - an upstream body (53; 63) delimiting a high pressure chamber (56; 66) at the inlet of the pressure regulator (5; 6) and carrying an expansion seat (552; 652) disposed opposite an inner channel (542; 642) formed in an upper portion (54; 64) of the upstream body (53; 63); - a movable piston (52; 62) forming a regulation part on which the gas from the low pressure chamber (513; 613) press on one side and a first biasing member (58; 68) press on the other side, said first biasing member (58; 68) being compressed between the piston (52; 62) and the upstream body (53; 63) by biasing the piston (52; 62) towards the downstream body (51; 61); - a regulating flap (572; 672) cooperating with the piston (52; 62) and movable relative to the expansion seat (552; 652) between an upstream position for closing a communication between the low pressure chamber (513; 613) and the high pressure chamber (56; 66) and a downstream position for opening the communication between the low pressure chamber (513; 613) and the high pressure chamber (56; 66); wherein the piston (52; 62) has an inner channel (520; 620) passing through and having two opposite ends: - an open upper end, on the side of the downstream body (51; 61), facing the low pressure chamber (513; 613), and - a lower end (523; 623) which opens onto an intermediate chamber (524; 624) interposed between the upstream body (53; 63) and the downstream body (51; 61); and wherein the pressure regulator (5; 6) further comprises a tubular valve rod (57; 67) provided with an inner channel (570; 670), the valve rod (57; 67) passing through both the inner channel (520; 620) of the piston (52; 62) and the inner channel (542; 642) of the upper portion (54; 64) of the upstream body (53; 63), so that this valve rod (57; 67) presents: - an upper end; and - a lower end forming the regulating flap (572; 672) suitable for bearing on the expansion seat (552; 652); said pressure regulator (5; 6) being characterized in that: - the intermediate chamber (524; 624) is connected to a leakage circuit; - the upper end of the valve rod (57; 67) is flared and is forming a discharging flap (571; 671) suitable for bearing on a discharging seat (521; 621) provided at the upper end of the inner channel (520; 620) of the piston (52; 62); and - a second biasing member (59; 69) is compressed between the piston (52; 62) and a plate (573; 673) secured to the valve rod (57; 67) and biasing the valve rod (57; 67), relative to the piston (52; 62), in the direction of pressing the discharging valve (571; 671) against the discharging seat (521; 621). - a static downstream body (51; 61) delimiting a low pressure chamber (513; 613) at the outlet of the pressure regulator (5; 6); - an upstream body (53; 63) delimiting a high pressure chamber (56; 66) at the inlet of the pressure regulator (5; 6) and carrying an expansion seat (552; 652) disposed opposite an inner channel (542; 642) formed in an upper portion (54; 64) of the upstream body (53; 63); - a movable piston (52; 62) forming a regulation part on which the gas from the low pressure chamber (513; 613) press on one side and a first biasing member (58; 68) press on the other side, said first biasing member (58; 68) being compressed between the piston (52; 62) and the upstream body (53; 63) by biasing the piston (52; 62) towards the downstream body (51; 61); - a regulating flap (572; 672) cooperating with the piston (52; 62) and movable relative to the expansion seat (552; 652) between an upstream position for closing a communication between the low pressure chamber (513; 613) and the high pressure chamber (56; 66) and a downstream position for opening the communication between the low pressure chamber (513; 613) and the high pressure chamber (56; 66); - an open upper end, on the side of the downstream body (51; 61), facing the low pressure chamber (513; 613), and - a lower end (523; 623) which opens onto an intermediate chamber (524; 624) interposed between the upstream body (53; 63) and the downstream body (51; 61); - an upper end; and - a lower end forming the regulating flap (572; 672) suitable for bearing on the expansion seat (552; 652); - the intermediate chamber (524; 624) is connected to a leakage circuit; - the upper end of the valve rod (57; 67) is flared and is forming a discharging flap (571; 671) suitable for bearing on a discharging seat (521; 621) provided at the upper end of the inner channel (520; 620) of the piston (52; 62); and - a second biasing member (59; 69) is compressed between the piston (52; 62) and a plate (573; 673) secured to the valve rod (57; 67) and biasing the valve rod (57; 67), relative to the piston (52; 62), in the direction of pressing the discharging valve (571; 671) against the discharging seat (521; 621).