The word is BRAKE.

I assume you mean that all four bulbs test okay.  If that is true, then
use a tester, either a test light or a voltmeter, and go to the brake light
switch.  Have someone press the brake pedal while you check at the switch
to see if you get voltage.

If you don't get voltage, move back from the switch toward the battery and
check for broken wires, blown fusible links, fuses, etc.  It would be best
if
you had a schematic to help you find the failure.

If you do get voltage, then there is a bad connection or broken wire between
the brake light switch and your brake lamps.  There is no simple way.  You
will
just have to trace it out until you find it.