I can't remember what the hoses on my old beast look like at the back.. so I can't think of the one you need...
If you mean the one that passes under the trailing arm mount (it connects the metal hose that's attached to the trailing arm to the metal hose that comes from the brake compensator valve) then you should be able to find replacements easily at a motor factor or on eBay.
Look at this one:
If there's another hose and eBay can't turn it up.. then look for anyone who can make hoses (search for Hel or Goodridge). I once had to make a custom hose for my 33 (anyone who had one might remember the long hose under the car). All you need to do is measure the hose length and tell them what fittings you've got and someone will be able to make you a braided steel one for not toooo painful.