Chances are you have a diaphragm problem. Do you know where your electric valves are located? If you can find the valves you can probably just swap the diaphragm out and it should do the trick. It might be best for you to buy / purchase the whole valve and just replace the solenoid, diaphragm, and bonnet assembly all at once. It might be cheaper than purchasing the parts separately.
Best of luck!
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Josh
Irrigation /Landscape Lighting / Pump and Well Specialist