rear brake pads on 2004 Chevrolet S10

do I need a tool to compress the rear brake caliper

Asked by for the 2004 Chevrolet S10
You need to rotate the piston back in to the set point. here is link to a repair info site for your truck that will help. Look under brakes and parking brake: I can often do it with out a special tool unless the piston is rusty, then you may need it or even a new caliper.