Are you talking about the L bracket on the frame where the line terminates and from there runs to each brake assembly as a hose? The other front brackets I recall are spring clips on the frame and the line just snaps into them.
I'll try to explain, assuming I am thinking about the same bracket you're asking about.
The L brackets fastens the brake hose termination to the chassis. The hose terminates at the brake end with a brake tubing nut and at the frame end with a special fitting that retains it to an L bracket so that the steel brake tubing doesn't vibrate or flex. This termination has a shoulder on the hose side that seats flat against the L bracket. Sometimes the bracket hole and the termination is keyed using a D shaped hole so they can only install one way.
There is a groove in this termination just after it extends through the mounting hole in the L bracket. This grove ends up above the L bracket on the Firebird chassis. A spring metal clip locks into the groove to retain the hose. It is similar to a "C" clip except it is rectangular with a U cut out. It pulls the shoulder of the termination tightly against the L bracket through spring action so it can't move. Some folks call this a "horseshoe" clip. It has a 1/8" turned up edge that you can use to tap it loose with a hammer and screwdriver. This edge faces upwards and outwards on the firebird. To remove it, tap it or pry it further towards the wheel. Be prepared with a new clip if the old one is rusted out. It's made of spring metal and it will hold the end of the hose firmly into the L bracket.
The steel brake line prepared with a double flare w/brake tubing nut fastens into the hose termination. The termination is prepared to accept a double flared tubing and brake tubing nut. Be sure to hold this termination with a wrench while tightening the brake nut down. It is easier to remove this termination from the L bracket while fastening the steel line to it.
Use a tubing line wrench with these tubing nuts so you don't squash them. They will easily go out of round if you use an open end wrench.
Do not substitute a C-clip or other home-made makeshift clip for this. The ends of the steel brake line must be firmly retained so they do not vibrate or flex as the suspension moves. If the steel line moves around, it will stress fracture and you will loose your brakes.
Unless you are installing premade lines, you need to have to double flare the brake tubing. I have had the best luck with bulk roll tubing that I cut and bent to custom lengths. The premade lengths are wonderful if you don't need to alter their length. They are very hard steel compared with bulk tubing so they don't take a double flare easily using the cheaper flaring tool kits. They are more difficult to bend and nearly impossible to put a decent on-center double flare. If you can buy a length that is close to your requirement, you can use up the excess length somewhere in the car with loops or U shapes etc. You can also splice two pieces together for a longer custom run.
If you are not replacing hoses (which include these terminations as part of the hose assembly) you do not need to loosen the fitting from the L brackets but it will make access easier if you loosen them. You hold the fitting still with a wrench and loosen or tighten the tubing nut on the steel brake line where it fastens to this fitting.
If you are bending your own tubing, disconnect the original and carefully remove it from the cradle part of the chassis. You should remove the portion of the engine mount bracket that has the line captivated in place so you don't have to thread it through with too much bending. That small bracket can be removed without causing problems. You can duplicate the bends on new bulk brake tubing if you lay them side by side and take your time. Be sure to install the rock guard (steel spring that is spiraled around the tubing) in the original places.
Tape the ends of the new line closed while routing them through the chassis so you don't get dirt and contamination into the brake system.
You want need the car as high as you can safely raise it in order to see the spring retainers. Some may be broken or missing too. If so, you can buy originals from any of the speciality restoration parts places or you can install a rubber lined clamp that fastens with a single screw. Don't omit any of the original tubing mounting points. They are important in preventing line failure.
