a differential loading fluid check valve , including a unitized valve body having a single service port in its sidewall for selective access to the interior chamber of the check valve . check valve subassemblies are installed and removed through the service port for valve assembly and service . the same valve body can be configured for either single or double check valve operation by adding or removing check valve subassemblies . each check valve subassembly includes a base with an aperture , a pivotally mounted clapper , and an overlying pivotally mounted cam arm , spring biased against the movable end of the clapper . an arcuate surface on the cam arm is engaged by the clapper as it moves from a closed position to an open position , providing a beneficial differential reduction in the pressure drop , or loading , through the check valve .