My experience in rebuilding Hysters and Clarks is this.
In my opnion you need the special tools (e.g. not hammer and chisel). A hook spanner is one place to start, but sometimes that's not enough. I spent considerable time machining special tools to engage all 4 of the slots in the cylinder caps with a long lever arm so I could open the caps without damage.
I bought all of my seals from Hercules (formerly Hercules Bulldog) and they wouldn't sell direct to me, so I used Motion Industries as basically a pass-thru.
If some of the seals are bad its likely they are *all* of the same age and bad. So even if you fix one issue there can be others.
I used .010" shim stock wrapped into a "funnel" to get seals nested into bores. The conundrum is they usually have to pass an internal thread which would just chew up the seal. The telescoping mast cyl on the Clark was the most challenging, I had to machine these small bands of either PVC pipe or a brazed piece of metal pallet strapping to pre-compress the U-cups so they could be slid into the bores.
It was a challenging process, a lot of labor, but the parts cost is minimal, everything is dry now and hopefully good for a long time. Be really careful with the mast when disassembling it..there are shear/crush/pinch points that aren't obvious when everything is normally assembled but can quickly become a guillotine if heavy parts of it suddenly fall down. Typical C-clamps and 2x wood blocking are your friends.
Forgot to mention its worthwhile looking at the rod choming, you really don't want rusty spots or flakes as those are like knives or sandpaper tearing up the seals. A ding is OK as long as its smooth, you can use a diemaker's stone (start with like hard Arkansas if no others are available) to carefully polish (and it will be a dulled polish from the mirror chrome) down any edges which catch your fingernail. Rusty spots are more problematic, they can be smoothed for awhile but eventually will rust again and start the process over. Rod rechroming is possible but expensive and sometimes its easier/cheaper to engage a cylinder shop to make new rods from chromed shaft stock.
