When you press the brakes and the car noticeably veers to one side, that pull is a safety issue worth chasing down. It almost always means one front brake is doing more work than the other. Here are the usual causes, roughly in order of likelihood.
A sticking or weak caliper
The most common cause is a caliper problem on one side. A caliper that sticks applied makes that wheel brake harder and pulls you toward it; one that sticks released brakes weakly and pulls you away. Look for uneven pad wear and see sticking caliper symptoms. A seized piston or frozen slide pins is the typical culprit.
A collapsed brake hose
A brake hose can deteriorate internally and act like a one-way valve - letting pressure to one caliper but not releasing it, or restricting flow so that wheel brakes late. This mimics a sticking caliper and is easy to overlook - see brake lines vs hoses. Age and internal swelling make a hose the hidden cause of a stubborn pull.
Uneven pads, contamination, or tires
Mismatched pads side to side, or a pad and rotor contaminated with oil or grease, will grip unevenly and pull. Always replace pads as an axle set. Not every pull is the brakes, though - uneven tire pressure, a bad alignment, or a suspension issue can steer the car under braking too, so rule those out if the brakes check out.
Do not ignore it
A brake pull can become dangerous in an emergency stop, so fix it promptly. Start by inspecting the calipers and hose on both sides, then the pads and rotors. Replacing a bad caliper or hose and fitting fresh pads and hardware as a set restores straight, even stops.