C5's have a pair of 2" pistons per caliper - giving an effective area of 6.28". Rule of thumb for a car like yours would be to run 70% of the area in the rear, giving a piston of 2.36" diameter. Stock 79-81 rear disc use 2-1/2" pistons, ours uses 2-1/4" versus the smaller S10 rear calipers and are drum-in-hat.
Find out what diameter piston a setup has first before buying - too small will be ineffective, too large cause excessive lock up.
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