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Air left in the brake lines after a repair, or drawn in through a leak or a reservoir run dry. Brake fluid is effectively incompressible and air is not, so pedal effort goes into squeezing a bubble instead of squeezing the pads onto the disc.

The symptom drivers describe is a soft or long pedal that improves if pumped — the giveaway, because pumping compresses the air temporarily.

Modern systems make this harder to fix than it used to be. ABS hydraulic units contain passages and chambers that ordinary pedal bleeding never reaches, so a car bled the old way after ABS work can keep a pocket of air indefinitely — which is why a scan tool that cycles the ABS pump during bleeding is often necessary rather than optional, and why a performance code can persist after a bleed that seemed to go fine.