Brake squeal is simply a high frequency vibration between the brake pad and rotor, the energy created from this is heard in the form of sound waves. Typically, this situation is usually encountered during low to mid energy applications of the brake. It can be caused by a number of factors;

Glazed pads or rotors (often caused by aerosol brake cleaners)

Inadequate piston retraction within the caliper causing brake drag (old seals).

Miss-aligned caliper (or bracket, as found on single action calipers)

Single Action calipers with bent [floating] pins or inadequate lubrication of the pins, not allowing the caliper to properly float.

Brake rotors with excessive run-out.

Brake pad backplates warped (often the culprit for spongy levers too¡­)

Bound up (or bent) forks or miss-aligned wheel centering causing the forks to bind, putting excessive pressure on the caliper and/or rotor. If involved in a front end crash, good idea to check run-out on axle too.

Buildup of road grime and brake dust on the piston seal area effecting proper piston retraction. When¡¯s the last time the calipers were rebuilt? Racers should do that annually.

Flush the brake fluid thoroughly at least once a year, more often in humid climates or if used in severe conditions¡­it¡¯s cheap insurance! Old contaminated fluid diminishes the overall performance potential of the brake system and can lead to more expensive problems.