A contoured check valve disc, comprising: a pivot portion having an aperture configured to receive a pivot rod (Pr) to allow the contoured check valve disc to pivot on a pivot point between a closed position and one or more open positions; and a check valve portion attached to the pivot portion and having two sides (S1, S2), a first side being configured to respond to a fluid flow in a first direction and move the check valve portion away from a fluid opening (Fo) to the one or more open positions depending on a fluid flow in the first direction to allow the fluid to flow through the fluid opening (Fo), and a second and opposing side being configured to respond to fluid flow in an opposite direction through the fluid opening (Fo) and move the check valve portion towards the fluid opening (Fo) to the closed position to seal the fluid opening (Fo) and stop fluid flow, wherein the first side of the check valve portion has at least two contoured portions (32, 34) with different disc contours defined with respect to the pivot point and a farthest point (Pf) away from the pivot portion, including a near-sloped portion formed nearest to the pivot portion, having a near-sloped contoured surface (Sn) curving away from the pivot point to a point of inflection (Pi) with a first curvature, and a far-sloped portion formed farthest from the pivot portion, having a far-sloped contoured surface (Sf) curving away from the point of inflection (Pi) towards the farthest point (Pf) away from the pivot portion with a second curvature that is less curved or bent than the first curvature, the near-sloped portion and the far-sloped portion combining to form a combined surface (Sn, Sf) having a contoured pressure/force distribution on the first side to respond to the fluid flow in the first direction and move the check valve portion away from the fluid opening (Fo), characterized in that a left-to-right curvature of the contoured check valve disc is greater nearer to the pivot portion and flatter nearer to the farthest point (Pf), wherein, in use, left-to-right is the direction parallel to the pivot rod (Pr). a pivot portion having an aperture configured to receive a pivot rod (Pr) to allow the contoured check valve disc to pivot on a pivot point between a closed position and one or more open positions; and a check valve portion attached to the pivot portion and having two sides (S1, S2), a first side being configured to respond to a fluid flow in a first direction and move the check valve portion away from a fluid opening (Fo) to the one or more open positions depending on a fluid flow in the first direction to allow the fluid to flow through the fluid opening (Fo), and a second and opposing side being configured to respond to fluid flow in an opposite direction through the fluid opening (Fo) and move the check valve portion towards the fluid opening (Fo) to the closed position to seal the fluid opening (Fo) and stop fluid flow, wherein the first side of the check valve portion has at least two contoured portions (32, 34) with different disc contours defined with respect to the pivot point and a farthest point (Pf) away from the pivot portion, including a near-sloped portion formed nearest to the pivot portion, having a near-sloped contoured surface (Sn) curving away from the pivot point to a point of inflection (Pi) with a first curvature, and a far-sloped portion formed farthest from the pivot portion, having a far-sloped contoured surface (Sf) curving away from the point of inflection (Pi) towards the farthest point (Pf) away from the pivot portion with a second curvature that is less curved or bent than the first curvature, the near-sloped portion and the far-sloped portion combining to form a combined surface (Sn, Sf) having a contoured pressure/force distribution on the first side to respond to the fluid flow in the first direction and move the check valve portion away from the fluid opening (Fo), a near-sloped portion formed nearest to the pivot portion, having a near-sloped contoured surface (Sn) curving away from the pivot point to a point of inflection (Pi) with a first curvature, and a far-sloped portion formed farthest from the pivot portion, having a far-sloped contoured surface (Sf) curving away from the point of inflection (Pi) towards the farthest point (Pf) away from the pivot portion with a second curvature that is less curved or bent than the first curvature, the near-sloped portion and the far-sloped portion combining to form a combined surface (Sn, Sf) having a contoured pressure/force distribution on the first side to respond to the fluid flow in the first direction and move the check valve portion away from the fluid opening (Fo), characterized in that a left-to-right curvature of the contoured check valve disc is greater nearer to the pivot portion and flatter nearer to the farthest point (Pf), wherein, in use, left-to-right is the direction parallel to the pivot rod (Pr).