Agree with greg, another possible cause of inop. brake lights is the turn signal switch. Yes even if the sig. lights work!
100% sure about the fuses? And do the 4 way flashers work on the rear? That info. will help.
Ok that tells me that the wiring and bulbs are ok. However it can still be the signal light switch that is causing your brake lights not to work! The brake light circuit goes through the sig. light switch before it goes to the rear lights!!! If yo check the switch like greg told you to do in the first answer, and it is ok, then the signal light switch is the only thing left!
Hope you get it fixed, may need a mechanic to check this out, hands on.