How did you check everything that you mentioned? Proper way would be with a volt meter for the brake switch, and replacing current bulbs/fuses with known working bulbs/fuses. Especially with bulbs, they may look okay but still be causing problems, and replacement bulbs and fuses are cheap.
Once those are ruled out as the cause, check for corrosion in the bulb sockets and make sure the contacts are all in good shape.
Next step would be to start tracing wires and look for anything that could cause a short or any broken insulation. Corrosion will cause large amounts of resistance on the wire and prevent the circuit form working correctly.