what should I ckeck to solve this problem?
my tail lights work, but my brake lights don't on 2008 Kia Spectra
Replace bulbs first- most bulbs do both functions (and directionals) so more than likely just blew out the filament for brake. If that doesn't fix then replace your brake light switch. Oh if you have a 3rd brake light up top and works for brakes than no problems with brake switch.