I tried blowing out my sprinkler system and the pressure blew right though the top. I unscrewed the top dome and found that the metal seal with a rubber seal had worn away, there was a thin piece of rubber on the seal. Unfortunately, there is metal piece across the middle of the opening, which has a downward facing spring and a metal rod in the center to hold the valve in place. I tried cutting a tennis ball in half, but, I had to cut the tennis ball to fit around the metal rod. It did not work and my alternative is to disassemble the unit. Any suggestions?
That sounds like the vacuum breaker valve that prevents back-flow. They are generally required by law in sprinkler systems. In 1996 our county started requiring spring loaded types. There are many designs . If you can't find a replacement part from the manufacturer , you may need to replace the valve. If you replace the valve , the city or county may want to inspect it.