Hi Duck, Kathy,
Welcome to the Hi-Lo Trailer Forum (HTF).
First, the HTF is not a parts source in the sense that stuff is sold here. This is a bulletin board comprised of folks who own Hi-Lo trailers, like yourselves. In that light, if a member is looking for a part, information, or a repair procedure unique to Hi-Lo's, those who have knowledge of a source for the part, information, or procedure typically step forward and share their knowledge.
Most of the mechanical parts found in a Hi-Lo are pretty generic and can be found at RV supply stores. Hard -to-find parts unique to Hi-Lo trailers can be found at either or both of the two following sites:
For Hi-Lo Parts - J & R Trailer Repair or
Home - The Official HiLo Trailer Parts & Service
You specifically asked about the hydraulic system: manual and parts. The trailer owner/operator manual should have a section that deals with correct operation and maintenance, but doesn't cover much more than that. The manuals, that I've seen, also include a simple trouble-shooting chart. The HTF has a number of procedures listed in the "Library," including a subsection dealing specifically with the lift system. Here is a link:
Lift System - Hi-Lo camper travel trailer forum Check for hydraulic system parts at the two sources listed in the previous paragraph.
I really recommend you spend some time browsing the HTF and using the search feature. Its very good and a huge mass of information is available here.
Jim