Brakes lock up

I have a 87, Nissan Pathfinder 4wd. 443,178 mileage .And about 4months ago the back brakes started to lock up. I thought it could be the shoes ,so i changed them out.i also noticed on one side the roder had been dug into by the brake shoe.and when i push the brake pedal to the floor it has no compression.unless i pump it. wat do you think could be the problem.I could really use the advice.