Brakes squeal for two reasons. The first cause of squealing brakes is movement of the brake pad within the the braking assembly. The second most common cause of squealing ir worn out brake pads.
Well typically squealing brakes are caused by worn pads rubbing metal on metal. Check your well to make sure that the have not already worn the rotors or else they will have to be turned or replaced.
Because, like any other car in its condition will react, it needs break fluid. Low levels of break fluid will cause the brake pad to grate on the brake base. Metal on metal friction will cause a squealing noise.
the brakes are squealing because your brakes are bad on it. the pads are almost gone so you should go change them before it does more damage to your rotors