Question: Why are some brake caliper pistons plastic (phenolic resin) and some are steel?
Answer: The plastic material is actually phenolic resin. This high strength man-made material has several advantages over steel brake caliper pistons. The first advantage is corrosion resistance. The material will not react with water and salt and rust. But, if the brake fluid is acidic, it can damage the piston over time. The second advantage is heat resistance. The phenolic piston will not transfer as much heat to the brake fluid when compared to steel pistons.
When the engineers design the brake system they design the system with the piston material and brake pad in mind. The package of the piston, shim, backing plate and friction material are engineered together. If the original caliper piston was phenolic, the replacement caliper needs to have a phenolic caliper. If you buy a replacement and it has a steel piston instead of a phenolic piston, return it.
The one thing that can cause a failure of a phenolic or steel piston is a damaged piston boot. If the boot is missing, torn or not properly seated on the caliper or piston, corrosion on the surface or dirt caked on the surface of the piston, will scrub back and forth on the piston bore seal every time the brakes are applied and released. Before long, the seal will lose its ability to hold pressure and the caliper will start to leak brake fluid.
There are shims that can prevent heat and vibration from being transferred into the caliper pistons. NRSBrakes.com PISTON CUSHION is packaged with some of their brake pad sets is designed to be inserted into the inner bore of the piston.