guessing its the little pipe that goes into the rear brakes (after the rubber section)...all youve gotta do is wd40 the connections (bleed nipple especially) dont force anything
it may just need a brush down...if not first check youve got the spanners that match
need one replacement brake pipe (any garage should be able to make you it up to suit -- depending on cost may be just as cheap to let them do it), and a brake bleed tube (little rubbery pipe that fits over the brake nipple).
assuming you do it yourself...
if the new pipe is straight you'll need to shape it to match the old one (can be done by hand -- do this carfully and smoothly so you dont kink it or if you can borrow the pipe bender tool even easier though overkill i reckon).
Once shaped undo the connections on the old pipe, remove and replace it with the new one (tighten connections)...then bleed the brake, get another person to work the brake pedal a few times then hold the brake on...fit the rubber tube onto the nipple and undo (collect the lost fluid in a jar) ...if it starts to slow then lock it back down, and get the helper monkey
to work the brake pedal again, repeat until you get constant stream of liquid (shouldnt take very long as youre only filling the new little bit of pipe up). Tighten everything down.
Test the brakes, check for leaks...all good?...Job Done.
Dont drink, etc etc any of the brake fluid...were rubber gloves if you wanna keep it off your skin, and wash your hands properly afterwards cos its not nice stuff.