A dual chamber coolant reservoir having a single vent neck. The coolant reservoir is for an internal combustion engine cooling system wherein the reservoir housing includes a first chamber and a second chamber formed integral thereto. A vent neck includes an aperture for accessing the first chamber with either a two or three o-ring cap to maintain pressure within the first chamber. Means for venting the second chamber when coolant exceeds a predetermined pressure level and a means for venting the first and second chamber when said cap is moved from a closed position to an open position. An inline pressure relief valve and check valve providing pressure relief and air displacement.

A safety cap device for controlling a pressure in a radiator includes a valve seat provided with a valve receiving space in which a first communicating hole is formed. A negative pressure valve is provided movably in the first communicating hole. A seal is disposed on a receiving surface of the valve seat and disposed movably with the negative pressure valve. The seal has a second communicating hole formed at a central portion thereof. A positive pressure valve is disposed on the seal and has a third communicating hole formed at a central portion thereof. A cap covers an upper portion of the valve seat. A spring is interposed between the positive pressure valve and the cap. A fluid receiving part is in the valve seat to prevent cavitation caused when an internal fluid expands.

A radiator cap includes a top cover unit, an intermediate seat unit, an output valve unit, and an input valve unit. The top cover unit includes cover body, a stem, and a plurality of positioning member to be fastened to a radiator opening. A washer surrounds the stem. The intermediate seat unit includes a connecting tube and a connecting seat connected to each other. The output valve unit includes a valve seat connected to the connecting seat. The valve seat has hook portions engaged with guiding recesses of the connecting seat. A valve disc is connected to the valve seat. A first spring is disposed between the valve seat and the connecting seat. The input valve unit includes a pin rod, and a second spring to bias the pin rod to thereby abut against the valve disc so as to close a valve hole.