Does anybody run the big GM calipers like the 76-78 Eldorado .I have all the hardware for a complete disc brake set up for the rear but I heard that the big caliper is to much brake for a BB Chevelle and is more suited for big trucks and old full size cars.Aparantly some guy had to run 2 proportioning valves to keep the tires from locking up on his car.
I will be running the C5 brakes up front.

I use those on the rear of my '72 with the stock master and front calipers and a 4 wheel disc prop valve and it works great. In fact when I installed this setup I went by the auto parts store to have them look up the info on the Seville caliper & master bores. The fronts on the Seville are the same as the originals on the Chevelle and so is the master bore. The rear calipers are also identical in bore to the metric style rears used on the late '70s WS6 trans am. I like the fact that the bigger calipers use the same pads as the front calipers. I have played around with different front/rear setups before getting it right. IMO it's best to use matching style setups, rather than try to fix everything with an adjustable prop valve. You may be better off using a rear setup from an LS1 Fbody, which also has a drum style parking brake. This means you dont have to use the parking brake to keep the rear calipers adjusted.

For the F-body LS1 set up,Is there any modifications needed (other than the E-brake) or is it a direct bolt on with the right back spacing, or do you have to shim the backing plate ?

hit the "search" button and find all the answers..

I did the LS1 brakes on my 65 Chevelle and had to shim the backing plate to make it all line up. Used some grade 8 washers and it took about two per bolt to line it all up. HTH.

Alex

Does the LS1 Trans Am have the same brake system ?(as in the same GM parts #)

I have the large GM calipers on the rear. Yes the piston area is very large and I went with a larger front caliper to match it better. I have a pretty good explination of my front system here: http://www.buffnet.net/~airrj/CBB%20Construction.html

My rear calipers however are just front calipers off of a late '70 Camaro. In other words, they have no e-brake in them. The Caddy systems with the e-brakes are much more expensive and I am installing a Ford 9 inch rear which I can easily run a driveshaft mounted e-brake.

As for the completed system, I have run several Autocross events and had a chance to do some brake testing on a closed course and my front to rear balance is very good. As it sits I have an adjustable proportioning valve in the rear line, but it is full open. And with a fresh set of Hawk HP+ pads the performance is outstanding for the cost. Pedal feel is great and I can modulate the brakes with ease.

Good Luck.