How long does it drain out? 15 Min? an hour? does it always drain out 24/7?
If it's only draining out 15min or so
Sprinkler systems are a gravity based system.. It's the lowest head at the end of the line and it is draining.. It will continue to drain until the zone has reached atmospheric pressure.
You can install a SAM Sprinkler head ..
SPRAY ,
ROTOR ... To help prevent this from draining .. About 8' head it has ability to stop water draining.. The downfall is the next head in line will drain until it reaches atmospheric pressure.