Your rear drum brakes squeal for a number of reasons. One reason is brake dust that has accumulated in the brake drum. When you apply the brake, the pads inside the drum come in contact with the brake dust, which causes the squealing. Another cause is the absence of contact between the drum and shoe. These 2 parts should come into contact when the brakes are applied and if this does not happen, squealing occurs. Brake shoes could have most of their lining worn away and thus be causing squeal by metal to metal contact of drum and shoe.