Abstract: A cover seal system for a valve housing with an access port, such as those found on backflow prevention devices. The invention generally includes a generally U-shaped gasket positioned on a peripheral edge of an access port located in a lateral wall of a valve housing and an arcuate cover for closing the access port. The geometry of the arcuate cover creates a clamping force which is more uniform across a surface of the gasket enabling the cover to seal at an elevated test pressure. The gasket is made relatively hard to prevent extrusion from the joint between the valve housing and the cover under elevated pressures.