A low brake pedal is caused by a number of issues. There could be a break in one of your brake fluid lines. The master cylinder could be leaking. On a front disc/rear drum-equipped car the brake cylinders could be leaking as could, the emergency brake cylinder or the front caliper assemblies could be leaking on a front disc brake vehicle. Perhaps the most common reason for a low brake pedal is wear on the discs or disc/drums. The only way you can tell where the issue lies is by parking your vehicle on some white cardboard overnight and looking for gray leaks. If there are no leaks, then it’s brake wear.
For more info, see 2 Signs a Brake Drum Needs Resurfacing