These are the parts that convert pedal travel into pressure, and they fail in ways that are easy to misdiagnose.
A master cylinder that is bypassing internally will let the pedal sink slowly at a stop light with no visible leak. A wheel cylinder that is seeping will show as a damp patch inside the drum and as pulled braking on that side.
Bench bleeding a master cylinder before installation is not optional; trapped air in a new cylinder will make the pedal feel spongy no matter how long you bleed at the wheels. Vehicles with ABS may also need a scan-tool actuation cycle to clear air from the modulator.
Reservoir and cap listings are separated because the cap contains the fluid level sensor on many applications, and replacing the cap alone fixes the warning light without touching the hydraulics.
