Do the easy stuff first. Check the pad pins for wear or signs of problems, check the caliper locating pins if the grease is dry, pull the pads an compress the lever carefully making sure the piston isn't stuck do not let the piston extend to far out of the piston bore. With pads out a light spray of wd 40 around the piston/caliper piston bore can help free up a stuck piston.

Digging deeper replace the old brake fluid it helps get rid of sponginess. Using a mityvac makes this job clean &fast.