A fluid flow valve is disclosed which includes a fluid flow assembly, a piston diaphragm assembly, and a reset plunger pin assembly. These three assemblies are configured to engage one another such that, when used in conjunction with a pressurized water source, they will split the pressurized water source into two portions, the one portion defining a main fluid flow pathway through the valve and, a much smaller proportion, defining a metered fluid flow pathway through the valve. The piston diaphragm assembly has a diaphragm or other equivalent structure for defining a metered fluid flow chamber, which metered fluid flow chamber is fluidly engaged to a pressurized water source through a metering channel in a piston. A reset plunger meter pin assembly engages the metered fluid flow or piston chamber of the piston diaphragm assembly and can initiate a timed flow of water or other fluid into the metered fluid flow chamber, which timed flow controls the flow of fluid or water flowing through the fill valve.