What part of the brake chamber are you expecting to leak? I ask because I've built a couple of chambers on a CAT loader some years back and from what I remember about them the condition of the rod makes no difference because the only air pressure in the chamber is to the rear of the diaphram and not on the front side where the rod is. That being the case, the only reason air would leak around the rod would be because there is another problem internally causing the air to leak around the piston seal.
With all of that said, again based on what I remember about the chambers I rebuilt (the ones with a ring of bolts around the end of the chamber), the stoke on the chamber CAT uses is longer than the stroke on a typical chamber like your talking about using. However if you can find a chamber that will match functionality, stroke, and mounting dimensions, there is no reason that something other than the actual CAT part won't work the same as the CAT part.