If you are seeing issues with the water continuously flowing with a full flow from your valve, then follow these steps:
First, confirm that the valve is installed in the right direction. The flow arrows on the valve should be pointing in direction of the water flow.
If the Valve is installed correctly, make sure the valve manual on/off lever/bleed screw is in off position.
If the valve is turned off manually, turn off the water and remove the solenoid. Make sure there is a plunger in the solenoid and it pushes up and down easily. Install the solenoid back in the valve and only screw down finger tight.
If the valve is still open, check the diaphragm for debris. Also, check for noticeable warping or pitting of rubber. If debris is found, clean out and rinse the diaphragm with cool water. Place the diaphragm back in and test the valve. If this does not resolve or if there is noticeable damage to the diaphragm, it will need to be replaced.
If you are seeing issues with the water seeping out of the lowest sprinkler head, you will need to replace the diaphragm in the valve.
Another thing to note if you are seeing this issue is that if your valve will not turn off, you should try unplugging/removing power from your timer. If the water stops, then that means that your timer is still sending voltage to the valve. If the water continues to flow, then the issue is stemming from your valve.