Is there an option to replace the vacuum seal in the main valve of a sprinkler system?
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.