Need to replace right rear brake tube on 1999 Buick century custom

only replace lines as any other repair is strictly illegal and dangerous

the leak can be from rusted steel pipes and fittings or failed /rotten rubber hoses

A little more detail? If there is no fluid coming from the bleeders, is there fluid present at the hose inlet of the caliper? Need to verify fluid present at rear or not...

The brake line needs to be replaced or repaired.

Brake lines by rear door on drivers side are a common failure point. lines under door failing can cause a total brake failure. Just replace and bleed the lines.

Sounds like you may have a hydraulic leak somewhere. Check the rear wheel cylinders and front brake hoses going to the calipers.

It probably brake fluid - thats the only fluid that's near the wheel/tires. You may want to
get brake line service before traveling anywhere in your car, since a large brake fluid leak disables the hydraulic braking in your car, leaving the parking brake as the only functional braking system.

contaminated with what? if it has had something other than brake fluid added yes it can get expensive would need to be flushed out and probably calipers and wheel cylinders replaced master cylinder replaced anything with rubber seals would need to be replaced

You've got a wheel cylinder leaking. If you are not familiar with waking on brake systems. I would suggest that you take it to a qualified mechanic. Brake work is critical to the safety of the vehicle, especially when it involves more than just simple brake shoe replacement.

All of that is not important

You can not get any brake tubing from a dealer, even for a
brand new vehicle,never could

When 3/16" steel brake tubing rusts out,you need to purchase
tubing of different lengths, at the auto parts store.

You cut,bend,and flair the ends yourself,fabricate so to speak,
no other way.

You use only the 3/16 double flair connectors, if you can not make
an end to end run.

You start by removing the tubing near where it rusted through or leaks
You go to the nearest original connection,a wheel cylinder,abs module etc.
Then as you construct the tubing going back toward the master at the front,
you may want to cut it and flair the OE Tube and stop there.
There is no need to go end to end,and put yourself through all that stress.

Just replace the rusted area unless you can easily get to a line.

If your not going back to the maste,r which line doesn't matter
If your going end to end you will answer your question as you remove
the old tubing

If your doing several rusted lines for safety sake,do one at a time

Under the hood is the master cylinder. Look at this picture:
http://s2.photobucket.com/albums/y48/ZeeVert/Other%20Stuff/?action=view&current=CenturyBrakeCylinder.jpg
It's back by the firewall, driver's side, near strut tower.
It is a plastic reservoir with a black screw-on cap. The cap has yellow lettering and probably tells you what brake fluid to use, or check the owner's manual. The plastic reservoir will have some marks on the side for "FULL". Do NOT overfill. Do not mix different types of fluid. The brake fluid will be something like "DOT3" or "DOT2".