I need to know how to fix my brake line for a 1999 buik lesabre limited edition

I recommend getting a Haynes manual to follow when doing this repair (if you are going to do it on your own). These manuals have pictures, and very thorough instructions for repairs such as this, along with safety guidelines to make sure you are doing it properly. It will help you avoid mistakes and keep you safe on the road. I'll include a brief description of how to fix a break line, but I will probably forget to write something here, ad I don't want you to get hurt because I forgot to put in some piece of information -- so please buy the manual to make sure I have given you good information.
1. You'll need a pipe-cutter, a pipe bending tool and a pipe-flanger to do the job 2. Find the break line that is causing the problem and trace it from the breaks to the break Master cylinder. 3. Take a piece of steel pipe (standard break line) and use the pipe-bending tool to shape it to look exactly like the pipe you are replacing and use the pipe cutter to cut it to length (if you don't use a bending tool or a pipe cutter, you can cause a crimp in the line and your breaks will not work) 4. If you are replacing the entire line (recommended) remove the line from the breaks and drain off the fluid. 5. Flange (widen the pipe end) your break line at the end closest to the breaks so that it fits over the connector (like the previous break line was). 6. Connect the new break line to the breaks using a new connector (same type as the previous break line. Make sure that the rest of the break line is supported near the original line to keep from ruining the new line -- if it bends or crimps your breaks may not work. 7. Remove the old line from the mounting brackets one at a time, and replace it with the new one as you make your way up the line to the front of the car. Doing this one bracket at a time will help you make sure you are mounting it properly, and gives you a chance to inspect the mounting brackets as you go. If a bracket is bad (and won't secure the line) replace the bracket with a new one -- you don't want your new break line falling down and getting ripped off your car when you hit a pothole. 8. Once you get the line mounted all the way to the master cylinder, complete the removal of the old line, and attach the new one like you did at the breaks-end of the line. 9. Double-check all of your connections and make sure the mounting brackets are all tight, then bleed the break line to remove the air. (it's probably best to bleed ALL of the break lines to make sure you have not introduced a bubble anywhere -- this can keep your breaks from working properly) 10. Once the lines are bled, and everything seems to be in order, jump in the car and hit the breaks REALLY hard -- like you are slamming your breaks to save your life. Check the lines again to make sure they did not leak anywhere (especially at your connections where you had to flange the pipe). If there are no leaks, you should be good to go. If there is even a small leak anywhere in the line or at your connections you should go over the connections again to make sure they are secure. Once you have your lines tight, bled and have double checked that you can slam the breaks without causing a leak, you should be good to go.

I do not recommend doing this work yourself -- it is very difficult to do without a professional lift (as you will need space to work and jack stands don't give you much room). It is also a VITAL part to the safety of everyone in the car ... and if you don't have a lot of experience it is easy to miss something and put yourself at risk. A professional shop and get this job done in a lot less time, and with a lot less risk than an at-home mechanic, so it is probably worth the price. As I said before, the 10 steps above are a GENERAL OVERVIEW of the process -- not an exact science to follow -- I probably forgot to put some information there ... so PLEASE (if you are going to do the work yourself) get a manual to be sure you have all of the steps correct. I HIGHLY recommend that you take it to a mechanic, though (I do a lot of my own car repairs, but I don't trust myself with the break lines -- they are too vital to my safety ... and a pain in the neck to replace without the right equipment)