when there is fire in the building, the solenoid valve is activated but what happen inside the cylinder actally?
If the pressure in any one zone is different than the other zones it could be one of two reasons; either the pressure is high (usually due to one or more sprinkler heads not poping up), or its low (usually due to a leak in the pipes). Something that could cause low pressure in all the zones are an obstruction in the water line above the valves (unlikely), or a hole in the line above the valve (the water would be filling the valve box and causing damage to the lawn wherever it is located), or leaks in all the lines (again would be easy to identify from the excess water in the ground at the source of the leak). Lastly the valve itself could potentially be faulty. The solenoid inside the valve holds the valve open when electricity is applied across it. If the wires to it are damaged; if the solenoid is bad; or if the valve has any rust or dirt inside that could cause a lower than normal amount of water flow.
