A valve for draining contaminants from an air reservoir on a vehicle, comprising: a housing comprising an inlet, an exhaust, a control port, a valve seat, and a wall, wherein the inlet is connected to a drain of an air reservoir, and wherein the control port is connected to a vehicle's emergency brakes; and a diaphragm positioned within the housing, wherein the diaphragm comprises a fixed portion that does not move relative to the housing and a moveable portion, wherein the diaphragm is moveable between an open position, in which the inlet and exhaust are in fluid communication, and a closed position, in which the moveable portion engages the valve seat to block fluid from flowing between the inlet and the exhaust, wherein control port fluid entering the control port exerts a control port force on the diaphragm, wherein inlet fluid entering the inlet exerts an inlet force on the diaphragm, wherein a biasing force is exerted on the diaphragm, and wherein the diaphragm moves from the closed position to the open position when the inlet force is greater than the sum of the biasing force and the control port force, when the emergency brakes are released, the control port is pressurized to keep the diaphragm in the closed position, and when the emergency brakes are set, the control port is not pressurized and the diaphragm moves to the open position to drain contaminants from the air reservoir when the inlet force from the pressure in the air reservoir is greater than the biasing force; and a spring assembly positioned between the wall and the diaphragm, wherein the spring assembly exerts the biasing force on the diaphragm when the diaphragm is in the closed position and when the diaphragm is in the open position, characterized by that the moveable portion is generally planar when not tensioned and wherein when the diaphragm is in the closed position the biasing force of the spring assembly offsets the moveable portion a distance D from the fixed portion to place the moveable portion of the diaphragm in tension. a housing comprising an inlet, an exhaust, a control port, a valve seat, and a wall, wherein the inlet is connected to a drain of an air reservoir, and wherein the control port is connected to a vehicle's emergency brakes; and a diaphragm positioned within the housing, wherein the diaphragm comprises a fixed portion that does not move relative to the housing and a moveable portion, wherein the diaphragm is moveable between an open position, in which the inlet and exhaust are in fluid communication, and a closed position, in which the moveable portion engages the valve seat to block fluid from flowing between the inlet and the exhaust, wherein control port fluid entering the control port exerts a control port force on the diaphragm, wherein inlet fluid entering the inlet exerts an inlet force on the diaphragm, wherein a biasing force is exerted on the diaphragm, and wherein the diaphragm moves from the closed position to the open position when the inlet force is greater than the sum of the biasing force and the control port force, when the emergency brakes are released, the control port is pressurized to keep the diaphragm in the closed position, and when the emergency brakes are set, the control port is not pressurized and the diaphragm moves to the open position to drain contaminants from the air reservoir when the inlet force from the pressure in the air reservoir is greater than the biasing force; and a spring assembly positioned between the wall and the diaphragm, wherein the spring assembly exerts the biasing force on the diaphragm when the diaphragm is in the closed position and when the diaphragm is in the open position, characterized by that the moveable portion is generally planar when not tensioned and wherein when the diaphragm is in the closed position the biasing force of the spring assembly offsets the moveable portion a distance D from the fixed portion to place the moveable portion of the diaphragm in tension.