Product Description ZURN WILKINS 3/4"-850T Full Port Tapped Bronze Ball Valves are recommended for residential, commercial and industrial ape- plications. Ideal for throttling and balancing installations of low pressure steam, air, water and non-corrosive gas or oil. From the Manufacturer When the Wilkins 975 XL fails, sometimes it is because the diaphragm becomes brittle over time or gets a tare. The 974-43 is the diaphragm that creates a seal in the relief valve which gives way if there is a failure in one of the checks on the back flow preventer. The relief valve is crucial to the proper function of the 975XL backflow preventer and should always be in excellent condition.