Some people paint their calipers by themselves and skip the important rebuild process.
Why do pistons get sticky and seized?
This is the most common problem on cars that are not used daily.
While the car is not used, the fluid seal will stick to the piston leaving a fraction of the rubber on the piston which will restrict their free movement.
Calipers need an internal clean every couple of years.
I have seen calipers removed from a car with only 30k miles on the clock with pistons that hardly moved.
But because they didn’t leak the customers thought they were fine.
The truth is they weren’t performing as they were designed to.
Sticky pistons will wear your pads and discs quicker.
You will get less pedal feel, contact between pads with discs while braking will be milliseconds slower than it should be with sticky pistons.
When you’re braking from 100mph you will lose a couple of meters (at 100mph car does 44meters in one sec.).
Think about it; those meters could be the difference between a safe stop and a crash.
Please beware of some caliper repaint services which DO NOT strip and rebuild calipers.
Such operators tend to use wet paint instead of powder coating.
In this process, the dust seals and pistons are simply masked up prior to the repaint.
You will get shiny calipers – but with the same sticky pistons.
The powder-coating process includes sandblasting and paint-curing (in an oven at 200°C), so all internals needs to be removed.