I got a set of the 4 piston caplipers from a junk yard and when I took one of them apart to clean and rebuild I found a stuck piston in one side...I got it out and the piston has rust forming on the lower side of the piston...I checked the caliper housing it was stuck in and it seems fine--ie no scoring!

Now the question...Can I get a new piston and/or do I need to get the caliper housing replaced on the side it got stuck.

 

If there is no scoring Id imagine they can be reused. However, check with mazdaformance.com parts catalogue to see if you can get a new piston.

 

Yea, condensation will form on mostly the lower pistons because of small amount of water content that forms in the brake fluid. I use crocus cloth to clean up pistons, caliper bore. As long as the bore isn't pitted badly.