Try going in reverse and braking hard a few times, maybe they didn't adjust the rear drum shoes properly. They self adjust when you brake in reverse if they freed the self-adjuster star wheel .
Your rear drums have 2 pads opposing each other, one is a lead, other is trail.
If its still feels spongy, you will need to bleed out your brake fluid.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille