I have an '84 Z28 4 wheel disc...
is there such a replacement caliper?
or do you have to do the whole upgrade (1LE)?

No such item exists. Also, I don't feel that a twin piston caliper would make any improvement in pedal feel or fade. The inherent problem in using little rotors in big cars is that modulation becomes difficult and inconsistent. I bet that if our cars came with 1LE sized rotors and the same cast iron stock single piston calipers, our brakes would be pretty good. I just did the 1LE upgrade, and put in a 97 T/A rear end w/ 11.625 rotors, and it stops like an animal now. There really is no cheap solution to upgrading the caliper or rotor.

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-Jason M. 1991 Camaro Z28