Why does the brake lock during braking?

Answer 1

Gunk build up inside the brake lines, gunk get stuck right where the line has a bolt on clamp,fluid pressure will only go to the clamp on the hose to the brake caliber but not back to the master cylinder.Two ways to check :have someone press the brake pedel,take off master cylinder cover and have them release the pedal look at the fluid return into the reservoir,it should fill fast,if it's slow most likely the line,if no flow at all it's the caliber. Or jack the front wheels up press the brake petal when release spin the wheels if its hard to spin it's the lines,if no spin it's the caliber.Please note if your car is front wheeldrive take it out of gear regardless of make and model use Wheel Chokes and emergency brakes when checking and repairing!Do not bleed the brakes flushing is alot better.

Contaminated brake shoes or pads or an ABS (Anti Lock Brake) sensor, could cause one of the wheels to lock while braking.

Answer

If by "lock," you mean the wheel suddenly stops turning, there can be several causes:

In general, static friction is greater than dynamic friction. (Static means not moving and dynamic means moving) An example of that would be when you push something across the floor. The force needed to move the object generally decreases once that object starts moving. The same is true for your tires in contact with the road. When they are rolling, the surface of the tire is not moving relative to the road. When the tires begin to skid, they are moving relative to the road. Skidding actually slows the car less than braking just slightly below the traction threshold of the tire.

Locking due to mechanical problems: If a brake disk or drum is not uniformly shaped (warped), the braking force will not be uniform as the wheel rotates. In this case, you may find that the brakes pulse or even lock the wheels when traction is low. The solution is to resurface or replace the disk or drum.

Answer

You have a pinched brake line. Inspect them all.

Answer

Pull the wheels and look. I would expect to find a leaking wheel cylinder(s). When they leak, the fluid gets on the shoes and cause a lock-up situation. As you drive more the fluid spins out and the lock up is less severe. Are you losing fluid?

Answer

Well if you pedel goes almost all of the way or does go all the way to the floor then the master cylender is bad.

you might have a leaking wheel cylinder or a caliper that sticks and does not release properly. or if it is panic braking, its just because you may not have ABS.