If you want to try rebuilding one yourself it is pretty easy. A good Gates brand kit is about $30 off of rock auto. Do a search for Gates 350440.
Get the box bare, then remove the four bolts that hold the sector shaft cap on, remove the cap. Next pull the sector shafts out of the box, you may have to tap on it a bit with a dead blow. Next grab a pin punch 1/8"ish, and drive it into the hole (that may be full of crud) that is on the perimeter of the casting where the piston cap retaining ring is. This will knock the retaining ring out of the groove so that you can remove the retaining ring completely. Next turn the input so that it pushes the piston twards the endcap. If you keep cranking it should pop the endcap out, making is so you can pull the piston out and all the bearings from the ball screw will fall out everywhere (do this in a sheet pan or something similar to make sure you don't have a bearing roll off to oblivion). Next measure the protrusion of the input shaft "nut" You want to put it back about where it was. After you measure it knock the lockout loose, then use a spanner to remove the input shaft assembly. You can clean everything up and replace the sector shaft bearing and seal in the case. Pull the input "nut" off and replace the seal in it. Take the piston and remove the ball screw retainer(two screws), and the old Teflon seals. Wrap the edges of the piston with electrical tape and grease it up well with transgoo, then carefully stretch the new (warm) Teflon seals over the piston and into the grooves. After you get them installed wrap them with a layer of tape, then a layer of shim stock, then clamp with a hose clamp until you are ready to install the piston. Re-install the input assembly to the same protrusion as original. Take the clamp off the piston, grease it and the case up with transgoo and carefully insert the piston into the case (you do not want to pinch the Teflon seals). Next you get the piston far enough into the case and clocked around so that you can access the ball screen retainer from the hole in the top of the case where the sector shafts cap goes. The next part is the assembly of the ball screw. This part can easily be screwed up, you must be deliberate. I normally have the box with the mounting ears in a vise, the input to my right, the piston cap to my left. The ballscrew is right hand thread. I normally feed the balls into the left hole on the piston. The ball bearings are often different colors (chrome and grey chrome). Alternate the colors as you put them in. As you put them in on the left hole you must make sure that they are going down on your side of the ball screw... NOT GOING OVER THE FAR SIDE. Once you get down to 10 or so left in your pile look at the other hole between every bearing install. Stop as soon as you see a bearing in the right side hole. Take the remaining bearings and use transgoo to hold the in the ballscrew retainer, then re-install it. Next clock the piston so that the sector shaft can be inserted and mesh with the teeth on the piston. Grease up the end of the sector shafts and carefully insert it into the case. Put the new gasket on the sector shafts cap and install it with the 4 bolts. Install the piston cap and retaining ring with a new o-ring. Go ahead and give the box a try on the bench, if there is any crunchy feeling, or anything loose inside you need to take it apart again, retrieve the bearings and try the ballscrew assembly again. You can tighten up the sector shafts play with the adjustors on top of the cap, and can also adjust preload on the input if need be.
