A water hammer arrestor and drain trap primer valve includes a hollow pipe having a closed end, a discharging end, an inlet, an outlet port connected to said discharging end and attachable to plumbing tubing, an inlet port connected to said inlet and attachable to plumbing tubing, an insert plug within said pipe positioned at said discharging end and at said inlet, said insert plug substantially tubular with opposing flats, and having a longitudinal cylindrical bore and a latitudinal threaded through bore, a port bolt within a portion of said insert plug and a portion of said inlet port, said port bolt substantially tubular and having a longitudinal cylindrical bore and a latitudinal through bore distributor aligned with the insert plug longitudinal cylindrical bore, and a check piston within said pipe and slidingly engageable with said insert plug and said outlet port wherein said check piston engages slidably responsive to water pressure changes at said inlet port and an integral air gap outlet.