A brake pedal that goes down too far before the brakes engage - a long, low pedal - is both annoying and a sign something needs attention. Excess travel means the hydraulics are taking up slack they should not have. Here are the common causes.
Air in the brake lines
The most common cause is air in the system. Air compresses, so the pedal travels farther before the fluid finally builds pressure - see spongy pedal causes. This usually comes after a fluid change, a repair, or a leak, and is fixed by properly bleeding the brakes - see brake bleeding basics.
Worn pads or rear brakes out of adjustment
Badly worn pads let the caliper pistons sit farther out, adding travel until they take up the gap - check pad thickness, see minimum pad thickness. On cars with rear drums, out-of-adjustment or worn rear shoes are a classic cause of a low pedal, since the self-adjusters are not keeping the shoes close to the drum.
A leak or failing master cylinder
A fluid leak lowers pressure and lengthens travel, and a dangerous version is a pedal that keeps sinking - see when a soft pedal is urgent. A master cylinder bypassing internally past worn seals also gives a long, sinking pedal even with no external leak.
How to fix it
Diagnose in order: check fluid level and for leaks, inspect pad and rear-shoe wear, then bleed the system to remove air. If a thorough bleed and good pads do not restore a high firm pedal, suspect the master cylinder. Fix the cause, then keep the system solid with fresh fluid, quality pads, and hardware.