Ok suzi,
you say they replaced the center seal 1yr ago?
ok that might be a place to look again.
The way the hydraulic system works on the mast is it is an equalizing type system, when you actuate the lift it sends fluid to the cylinders and if all is well the primary cylinder (center) is supposed to raise first, when it reaches it's max height and bottoms out the secondary cylinders take over. If there are no restrictions in the system this is how it is supposed to work.
If your center channels are raising prematurly then the center cylinder is restricted for some reason causing the fluid to divert to the secondary cylinders prematurly.

There are 2 things i would check
First of all i would have the primary (center) cylinder rechecked to make sure the packing in the gland nut isnt dry or has blown out and is causeing a bind against the rod.
Or even a small possibility that they installed the incorrect seal kit and it's just too tight causing the center cylinder to stick.

Second is there should be a small manifold block mounted about waist high on the back of the stationary channel between the mast and the cowl of the lift frame. (inbetween the mast and the truck) it should be mounted on the mast cross bar and has hoses or metal tubes running from it to the cylinders.
Pull the hoses and check inside this block for restrictions of any kind. If it has a check valve, check that for debis or being stuck.

Note: when they sourced the packing kit for the cylinder seal kit they should have looked on the cylinder, there is a cyllinder number group etched into the cylinder shell, usually somewhere on the side of the cylinder shell. You may have to clean it off to see this.

Hope this helps

Later,