If your vehicle is showing car pulls noticeably to the left or right when braking, here are the most likely causes and the parts you'll need to fix each one.
Rust, debris, or lack of lubrication can cause a caliper piston or its slide pins to stick. This prevents the brake pads from applying evenly on both sides of the axle, causing the car to pull toward the side that is gripping properly.
If brake fluid, grease, or oil leaks onto the brake pads on one side of the vehicle, those pads will lose their friction. The vehicle will pull sharply toward the opposite side where the pads are still dry and gripping normally.
If the brake system was recently serviced and not bled properly, trapped air bubbles will remain in the lines. Because air compresses (unlike brake fluid), the caliper with air in its line will apply less force, causing a pull.
Over time, the inner rubber lining of a flexible brake hose can break down and act like a one-way valve. This restricts fluid flow to the caliper, causing a delayed braking response on that side, or preventing the caliper from releasing after you let off the pedal.
When you brake, the vehicle's weight shifts forward dramatically. If the rubber bushings in the front suspension are torn or worn out, the braking force will cause the control arm to shift out of alignment, steering the car to one side.
A rare internal failure in the ABS hydraulic valve body can block fluid from reaching one of the calipers entirely. This leads to a drastic imbalance in hydraulic pressure and a severe pull when braking.