Here is an update. I ordered a seal overhaul kit, but when it got here it didn't include the damaged seal. I contacted the manufacturer and they said that they no longer feel that seal is required and no longer use it, so they recommended just removing it. I also asked them what type of lubricant to use for the internal seals, and what type of periodic maintenance should be done. They said that the cylinder is coated and additional lubricant should not be used because it will collect dust and dirt in the seals. They also said that no periodic maintenance is required, other than keeping the external piston clean.