You press the brake and the pedal feels low or soft, but pump it a second time and it firms right up. That specific pattern - soft first, firm on the second pump - is a classic symptom with a short list of causes. Here is what it means.
The most common cause: air in the lines
Air trapped in the brake hydraulics is compressible, so the first pedal press squeezes the air and feels soft; pumping compresses and stacks the fluid so the second press feels firm. This is the textbook sign that the system needs bleeding - see brake bleeding basics - often after a fluid change, a leak, or a repair that let air in.
Worn pads or excess rotor runout
Badly worn pads or a rotor with excess runout can knock the caliper pistons back slightly, so the first pump takes up the extra gap and the second is firm. Check pad thickness - see minimum pad thickness - and rotor condition; this version usually comes with other wear symptoms.
A failing master cylinder
A master cylinder with worn internal seals can bypass pressure, giving a pedal that sinks or feels inconsistent and may temporarily firm on a quick pump. Combined with a pedal that slowly sinks under steady pressure, suspect the master cylinder - see spongy pedal causes.
Do not ignore it
A pedal that needs pumping is telling you the brakes are not fully solid, and it 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.