I just looked at the Bendix site and they carry parts for the actual rotochambers. That said, I also did a search for CAT brake chambers and now I remember what your talking about. I did build a couple of them about 10 years ago. The lower part of the canister is made like a Rotochamber and the actuator rod your talking about being pitted is hollow. When you apply air to the bottom of the chamber the hollow shaft rises and releases the parking brake. Then there is another regular pancake type chamber at the top that actually applies the brake to stop. The rod it moves runs through the middle of the hollow one. One good thing about that setup is that as long as the leaks don't get so bad that they drop pressure far enough to kick the parking brake valve back out, then you can live with them and still not be worried about the actual service brakes working.