Disclosed herein is a pressure valve for use in any one of several cooling systems for different internal combustion engines also disclosed herein. The pressure valve is connected in series with a source of coolant and an engine coolant jacket for affording coolant flow from the source through the coolant jacket when the pressure in the coolant is above a first value and for affording continued coolant flow through the coolant jacket until the pressure in the coolant falls below a second value lower than the first value. The pressure valve comprises wall means defining a chamber having an inlet and an outlet including a valve seat, together with a valve member having a first surface movable relative to a closed position wherein the first surface sealingly engages the valve seat, which valve member also has a second surface facing away from the first surface. Means biasing the valve member toward the closed position are provided, together with a diaphragm having a portion connected to the valve member for proportionately related movement in common with the valve member, the diaphragm having a side surface positioned in spaced relation with respect to the second surface of the valve member so that liquid entering the inlet at a pressure above the first value is effective to displace the valve member from the closed position and so that the valve member remains displaced from the closed position until the pressure in the liquid entering the inlet falls below the second value.