Most rebuild kits don't include pistons. I've made slightly oversized pistons on a lathe at times, but that cut into the profit margin pretty severely! The rebuild kits have all the rubber parts such as cups, dust boots, etc, and usually come with a new spring and sheet metal cup expanders. Most wheel cylinder leaks are due to a slightly rough surface of the cylinder bore, or poor quality rubber in the cups. NAPA wheel cylinder kits are usually very good quality. Back when there were a lot of drum brake systems on the road, I had a stock of bulk wheel cylinder cups of various sizes, and if the rest of the small parts were good, I'd just hone the cylinders and replace the cups, instead of using complete kits. The cost of doing it that way was considerably less, and every penny I saved put more food on my kids' plates and kept a roof over their heads!
Jerry
