that "thick paste" is common, nothing dissolved, just the shoe dust combined with lots of different semi-liquid vapors: road spray, oil fumes, brake fluid .... that pic looks like most old neglected brake assemblies, including the dead spider
.... and yes, make sure the park brake mechanisms move freely, they easily get stiff enough to maintain a drag on the rear brakes even tho' you think the hand brake is fully released, your situation of good fronts, thrashed rears can be the result of towing any distance with the park brake sticking, and is one reason the brake shoes should be removed before towing - fires can happen
Bill
