Many manufacturers are now choosing to use phenolic caliper pistons rather than steel. Like any component the specifications and materials vary. There are the good and the bad. Cost can often be the prime reason why a caliper manufacturer will use phenolic pistons. Technicians need to be aware of this trend of using phenolic pistons. The pitfalls they can cause when doing a simple brake-pad change and not knowing the facts.

Phenolic Pistons

08/12/2024

Written by Darren Caldwell

Don’t “Swell” on the details

Vehicles equipped with phenolic pistons are more frequently experiencing a variety of brake problems, including shuddering and squealing. These issues are largely due to retraction problems associated with phenolic pistons. Over time this material becomes unstable and swells, causing the caliper to bind. What begins as a straightforward brake-pad replacement can quickly escalate into significant caliper problems when the phenolic pistons are pushed back into the bore. This swelling occurs because the brake fluid has absorbed too much water, often due to neglected maintenance. The issue becomes particularly noticeable when the pads wear out, leading the pistons to push beyond their limits. Only the bottom 10-15mm of the piston that is below the caliper seal is in contact with the fluid. This area swells. When pistons are pushed back, the pistons jam or at the least, are so tight in the bore that retraction issues occur.

Know the Facts

Phenolic pistons are often fitted to imported vehicles used in cold climates where salt is used on the roads to prevent snow build-up. In this application they can last longer than steel pistons. Phenolic pistons are mold pressed, heated and molded into shape. They are commonly a more cost-efficient alternative to aluminium and steel and used in various original-equipment (OE) applications.

In summary, phenolic pistons can be useful in certain applications but repairers need to be aware that they can be very problematic as vehicles get older. It’s also worth noting that phenolic and steel pistons have different retraction rates. This means both calipers need to be rebuilt.

Check out our huge range of steel replacement pistons by size here or search for your vehicle using our vehicle lookup.