hey all, over the years i have converted 2 GSs to rear disc brakes from donor cars - thought I would share my experience in case it helps anyone. First one, back in the 90s - found a set from a 1976 cadilac - they bolted right up to the 10 bold and fit within the 15 inch ralley with a small amount of griding on the caliper! They use the same caliper we use on our front stock disc brakes (I always thought this design was a great candidate for a aftermarket manufacturer - could produce kits for CHEAP and I think a lot of folks would purchase!). I did have to re-drill the rotors for our bolt pattern. A conversion I just did was from a 98 camaro. You can use any 93-2002 camaro or firebird rear set up, although they do differ a little from 93-97 and 98-2002 (93-97 are 1/2 inch in diameter smaller). This conversion WILL NOT fit in stock ralley wheels. I had to press the old bearings off the axels and press the caliper plates on with new bearings. With new bearings, I have less than $300.00 in a new set of 11.5 (12?) inch rear aluminum dual piston rear disc brakes.
Here is a good article from another member here that is also doing that. http://www.buickperformance.com/rrdisc.htm
