hydraulic - add pilot operated check valves

This would apply to all hydraulic landing gear systems. Install pilot operated check valves on the cylinders to prevent trailer drop in the event of failure of hoses or fittings between the solenoid directional valve and cylinders. The check valves remain closed until pressure to operate the cylinders is applied. The pressure to operate the cylinders opens the check valve to allow oil to move the cylinders. No pressure, no cylinder movement.

I assumed the hydraulic fluid moves in both directions - one way to extend, the other to retract. If so, I'd expect you'd need some type of bi-directional check valve. Is that what you've installed? Can you provide a link to the website that has info about the device you're using.

I have not added pilot-to-open check valves. I am suggesting to the factory that they consider adding one to each landing leg cylinder. It would be installed directly to the cylinder 'extend' port. It would recieve a pressure signal from the 'retract' port. The pilot signal would open the check valve so oil flow could leave the extend side of the cylinder allowing the cylinder to retract. With no pilot signal the check valve would prevent flow out of the extend side of the cylinder and lock it. The check valve would open normally to allow oil to flow into the extend side of the leg cylinder to extend the cylinder.
Here is an example from Sun Hydraulics:
http://www.sunhydraulics.com/PDF Files/Shortcut/USscut/USSCpocheck.pdf