A disc assembly for a valve member, the disc assembly comprising: a plunger and a movable disc coupled to the plunger; a partition plate and a fixed disc interposed between the partition plate and the movable disc, wherein the fixed disc and the partition plate each comprise a duct (6a, 6b), the ducts (6a, 6b) being in fluid communication and receiving the plunger and at least one of the ducts (6a, 6b) having a cross-section such that the plunger is configured to reciprocate along an axis determined by the cross-section and to reciprocate the movable disc relative to the fixed disc along the determined axis; wherein the duct of the fixed disc has an end that defines an interface aperture having an outer circumference, the interface aperture opening in the direction of the movable disc; characterized in that the fixed disc comprises a collar protruding from the fixed disc in the direction of the movable disc and looping about the outer circumference of the interface aperture of the duct of the fixed disc; the disc assembly also comprising a trough-shaped portion defined and bounded by the protruding collar, by the fixed disc and by the movable disc, the trough-shaped portion being in fluid communication with the duct of the fixed disc via the interface aperture; wherein the movable disc and the protruding collar each has a surface in mutual abutment with each other such that the mutually abutting surfaces of the movable disc and of the protruding collar form a seal about the trough-shaped portion; wherein the disc assembly is configured so that when a first pressure of a fluid is applied to the movable disc from inside the trough-shaped portion and a second pressure of a fluid is applied to the movable disc from outside the trough-shaped portion, the second pressure exceeding the first pressure, a force is applied to the movable disc that presses the abutting surface of movable disc against the abutting surface of the protruding collar of the fixed disc. a plunger and a movable disc coupled to the plunger; a partition plate and a fixed disc interposed between the partition plate and the movable disc, wherein the fixed disc and the partition plate each comprise a duct (6a, 6b), the ducts (6a, 6b) being in fluid communication and receiving the plunger and at least one of the ducts (6a, 6b) having a cross-section such that the plunger is configured to reciprocate along an axis determined by the cross-section and to reciprocate the movable disc relative to the fixed disc along the determined axis; wherein the duct of the fixed disc has an end that defines an interface aperture having an outer circumference, the interface aperture opening in the direction of the movable disc; characterized in that the fixed disc comprises a collar protruding from the fixed disc in the direction of the movable disc and looping about the outer circumference of the interface aperture of the duct of the fixed disc; the disc assembly also comprising a trough-shaped portion defined and bounded by the protruding collar, by the fixed disc and by the movable disc, the trough-shaped portion being in fluid communication with the duct of the fixed disc via the interface aperture; wherein the movable disc and the protruding collar each has a surface in mutual abutment with each other such that the mutually abutting surfaces of the movable disc and of the protruding collar form a seal about the trough-shaped portion; wherein the disc assembly is configured so that when a first pressure of a fluid is applied to the movable disc from inside the trough-shaped portion and a second pressure of a fluid is applied to the movable disc from outside the trough-shaped portion, the second pressure exceeding the first pressure, a force is applied to the movable disc that presses the abutting surface of movable disc against the abutting surface of the protruding collar of the fixed disc.