Cause and cure for fluid on back brakes of 1999 4 runner 4x4 how do you fix it?

It could be the brake cylinder in the rear drums, if they have not been replaced since new they do begin to leak all over the bands causing a reduction in braking power and causing premature wear on the front pads, if you do change them change them in pairs, don't do one side and leave the other intact cause the older one will bust due to the increase in pressure. Oh and change the bands because brake fluid will damage the bands if they are soaked in brake fluid.