Master Station config

My OS system is used to fill a buffer tank.
Water is extracted from the ground by means of well points, pipes sunk into the soil about 5.5m to 6m deep.
As you can not extract a large amount of water at one time, you need to run the pump for 15 min. then wait 30 min. for the soil to saturate again.

The problem i have is that during the filling the system start to suck air, i have a pump protection device that stop the pump if their is no flow for x duration.
This air is causing the solenoid valve that control the filling to not shut when the schedule has ended.

Is their a way to open my Master Station solenoid 20 seconds before “Tank Fill” schedule ends and 20 seconds after?
This will fill the lines and make the solenoid shut.

The only way i can see this work is to have a 2nd Master and put a delay on it.
The delay must be 880 seconds before and 20 seconds after.
I see that Master On Delay can only be set to 60 seconds.

I am also unable to assign my 2nd Master, my system is configured as a 16 stations.
Is their a way to change this?

Regards
Z