[DonRotolo]

With spring-return pneumatic cylinders I am familiar with, the usual practice is to extend the piston by pressurizing the cylinder (they usually have only one 'hole' for air) and then commanding your solenoid valve to "close", maintaining the pressure. You then open it when you want it to retract.

You might need a different kind of solenoid valve (compared to the KoP version) to do this.