Started by the Pedalwise team from official manuals, support pages and repair videos

A brake pad spreader, or piston press, is a flat tool that pushes a hydraulic disc brake's pistons back into the caliper. New, thicker pads need that room to fit over the rotor.

What it's used for

When you replace pads on a hydraulic brake, Park Tool says to press the pistons in with a pad spreader or with a flat, non-marring tool such as a tire lever (Park Tool pad guide). Its brake bleed guide also lists a pad spreader and piston press among the tools needed (Park Tool bleed guide).

What to look for

• A flat, non-marring surface that won't scratch the pistons (Park Tool).
• A width that suits your caliper. If you don't have one, a plastic tire lever works as a stand-in (Park Tool).

Using it safely

• Press evenly. On Shimano flat-mount hydraulic brakes, Park Tool specifically warns to apply even pressure to avoid damaging the ceramic pistons (Park Tool).
• Don't squeeze the brake lever with the wheel or pads out. If you do, the pistons come out again and must be pressed back in (Park Tool).
• Mechanical (cable) brakes have no hydraulic pistons. Park Tool says to back their pad adjusters out instead (Park Tool).
• After new pads go in, pump the lever until the pads meet the rotor, then spin the wheel to check for rubbing (Park Tool).

Guides that use it

• Replace disc brake pads
• Bleed hydraulic disc brakes
• Bed in disc brakes

Sources

• Disc brake pad removal and installation, Park Tool
• Shimano hydraulic brake service and adjustment, Park Tool