An embodiment providing one or more improvements includes a trap primer with upper and lower chambers separated by a flexible diaphragm which also interacts with a valve stem to start and stop the flow of water from the trap primer to the trap(s). The lower chamber contains a polymeric foam medium such as a foam ring containing closed cells containing gas or gases. In use, equilibration of water pressures in the upper and lower chambers causes distortion of the foam ring, resulting in equilibration of the gas pressure in the closed cells with that of the chambers. A decrease in line water pressure causes water from the lower chamber to be emitted into the traps.