LED brake light bulbs .

Define "doesn't work." Does ANYTHING work? Tail lights? Parking lights? Brake lights? No reason brake lights should not work, regardless of the flasher.

You should be able to "jig" the lamps up on a battery / clip lead and see if they actually work
Parking lights work no turn signal and no brake , bulbs work on battery. One side on the bottom of the bulb is brighter than the other post .