Al-ko Phenolic Hydraulic Caliper Piston
Al-ko caliper pistons are not interchangeable with others brand of disc caliper.
What causes hydraulic piston failure?
- Rubber seal degradation.
- Water ingress, as a result of a hot piston during the rapid cooling process.
- Hydroscopic Brake Fluid.
Furthermore, purchase other piston spare parts here.
There is a great article on how to correctly install a marine grease seal.
Finally, the piston seal installation is difficult. Couplemate uses a silicon-based lubricant on the outside of the piston before insertion.
This video is from a VW, caliper seal installation. It may be helpful.