You press the brake and the pedal feels low, but pump it a second or third time and it comes up firm and high. A pedal that pumps up is a specific, recognizable symptom with a short list of causes. Here is what it means.
Air in the brake lines
The classic cause is air in the hydraulic system. Air compresses, so the first press squeezes the bubbles and feels low; pumping stacks the fluid and firms it up - see brake pedal soft then firm. This usually follows a fluid change, a repair, or a leak that let air in, and is fixed by properly bleeding the brakes - see spongy pedal after bleeding.
Worn pads or excess rotor runout
Badly worn pads or a rotor with too much runout can knock the caliper pistons back slightly, so the first pump takes up the gap and the next is firm. Check pad thickness - see minimum pad thickness. On drum-rear cars, out-of-adjustment rear shoes do the same - see drum brake adjustment.
A failing master cylinder
A master cylinder bypassing internally past worn seals can give a low pedal that firms briefly when pumped but sinks again under steady pressure - see brake pedal travels too far. This one comes with no external leak and is worth diagnosing.
Do not live with it
A pedal that must be pumped is not fully solid and can worsen. Most cases are air and fixed with a proper bleed; some are worn parts or a master cylinder - see when a soft pedal is urgent. Diagnose the cause, then restore a firm pedal with fresh fluid, quality pads, and hardware.