A brake that locks up - one wheel grabbing hard, the car pulling, or the tires skidding under light pedal - is unsettling and unsafe. Lockup has several distinct causes depending on whether it is one wheel or all four, and whether your ABS is working. Here is how to sort it out.
One wheel locks: sticking caliper or contaminated pad
If a single wheel grabs or stays dragging, the usual suspect is a sticking caliper - a seized piston or frozen slide pins keeping the pad clamped - see sticking caliper symptoms. A collapsed flex hose can also act like a one-way valve, holding pressure at one wheel. Oil or grease on the pad or rotor makes the pad grab unevenly too; reading the pad wear pattern often reveals which.
All wheels lock too easily: hydraulics or a bad booster
If the brakes are grabby everywhere and lock under light pressure, the balance between front and rear may be off - a failed proportioning or combination valve can over-apply the rear brakes and lock them first. An over-boosted feel from a fault in the brake booster can also make the pedal hyper-sensitive. Old, grabby, or glazed pads contribute as well.
Lockup when ABS should prevent it
Modern cars use ABS to stop wheels from locking during hard stops. If a wheel locks and skids anyway, the ABS may be disabled by a fault - check whether the ABS warning light is on. A failed wheel-speed sensor is a common reason the system stops preventing lockup - see how ABS works.
What to do
Lockup is a safety issue - do not keep driving a car that grabs or pulls hard under braking. Inspect for a sticking caliper first, then hoses, pads, and the ABS system. Fix the mechanical cause and replace any contaminated pads and worn hardware as a set.