Leaking from the back assembly typically comes from a scratched piston or bad 8013013 seals.

 

Recently Installed Repair Seal Kit

 

If you recently installed a RSK, The 8013013 seals (flared seals) many times are not seated completely during the repair seal kit installation. When installing each one, be sure that they are down all the way by leading the thin edge of the seal with your thumb nail.