The placement of the hose can play a large role in draining your cold brew system. Make sure to keep the end of the hose lower than the bottom of the cold brew system.
If you ever stop the flow of liquid from the system using the ball valve, pinch the end of the hose in order to keep it filled with liquid. Doing this will help restart the siphon when you re-open the ball valve. If all of the liquid flows out of your hose while the ball valve is closed, simply make sure that the end of the hose is lower than the bottom of the cold brew system when you re-open the ball valve. If the flow doesn't start back up, you will likely need to prime the system in order to restart the siphon. Take a look at the video below
Resolving a Stuck Siphon In Your Cold Brew System