[Christ]

Whatever you do, do NOT simply cap the lines at the piston, unless you've removed the internal seal from the piston. Air/fluid needs to be able to pass from side to side of the piston face to allow proper motion, else you'll be building pressure on each side of the cylinder [which isn't necessarily a bad thing, but in this case, you don't want it.]

If you take the rack apart, you really don't actually need to completely remove the piston. Removing the seal and/or drilling some holes through the piston face is more than plenty to allow effortless manual steering. Often, the energy spent removing the piston is energy wasted because the 'problem' can be 'fixed' so easily in other ways.

Once you've looped the lines at the piston, you can simply add plastic caps to the lines at the rotary valve, as there is no pressure differential inside the rotary valve without fluid moving through it. If you feel safer, some threaded plugs or even the original lines pinched off will work fine, also.