The 1967-68 disc brake option used the four piston Girlling caliper off of the Corvette with a special one off wheel hub. From 1969-'71 the factory changed from the expensive Girlling brake system when they made a special set of calipers and thicker rotors that where larger than those used on the smaller cars because they didn't know if the bigger car would stop safely with the smaller caliper and were afraid to risk it. Once the single caliper disc brake system introduced across the GM line in 1969 had been out for a few years GM felt the risk to their profits would be low enough to substitute the smaller brake caliper and rotor used on their lighter cars on the Impala. It was done to reduce the number of parts in inventory to save money. By 1972 all of GM's sportier cars where using 15 inch wheels if they had disc brakes installed. My mom's 1968 Caprice came with disc brakes and 15 inch Corvette Rally style wheels.