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.