Inspect for worn or loose wheel bearings.

Replace the wheel bearings.

Inspect for a broken or a sagging spring.

Replace the spring.

Inspect for a leaking wheel cylinder or caliper.

Replace the wheel cylinder or the caliper.

Inspect for warped rotors.

Replace the rotors.

Inspect for an incorrect or an uneven caster.

If the caster is beyond specifications, check the frame and repair it as needed.