Hey Eric, 
   Just wondering what ever came of your hydraulic problems. I'm very interested in knowing what you come across as you go through this process, as it sounds like I have the very same thing going on with my Super 55. -Mine is also an early model, a 1955, S/N 13,142, SPEC 15-0005.
I am about to change my hyd fluid once again, and do the filter with it this time. I will also try running it in pos control, as John suggested. I still need to locate a stand-pipe kit, but John was looking for me, I think? 
If anyone has found one, please let me know. It looks like with the kit, there is a change with the breathers too.....?
Thanks!
Patrick