Slave cylinder? Not on brakes guys. There is no need for a slave cylinder when there is no need for a hydraulic connection or when a mechanical connection is possible, eg brake pedal to the master cylinder, you know, the brake thing with the brake fluid resevoir on top of it? I hope this clears that up. But yeah, check for a leaking rear brake cylinder, as a few drops of brake fluid missing could cause spongy brakes as liquids are not compressible, though most fluids are (100% of gases).

Short story long, check for leaks under the rear drums and along the brake lines.