3. Water leaks from under the plastic cap on top of the valve

The vacuum breaker has begun to leak. This happens when the plastic cap is ruptured by freezing, this often occurs when the hose is left on the valve, or the rubber seating surface needs to be replaced. 

This vacuum breaker threads out of the valve, and the replacement threads into the valve. 

The parts required for this is a C-144KT-806 Vacuum Breaker Replacement Kit