there is no filter on the 87-97 f-super duty/f450 trucks,hydroboost system.
i once read an article (don't recall where or what now,but i recall it was from ford,like a tsb or something and i think it was talking about the newer trucks)
where it said not to install a permanent filter but if one suspected dirty fluid issues,one could flush and install a filter on the return line for i forget how long but like a ride or so,or 20 mins to clean and filter the fluid but then to remove the filter.
that said,i installed a filter on the return line on the hydro i installed for chip truck (my f250)
Chip Truck w/ Specs & Mods
and iv left it there for years now without any side effects.
my f450 remains oem without one.so you can make your own choice as to install one temp or long term but adding one is very easy.simply install it in the return line before the res.