I'm no sparky but it sounds like some sort of shorting out going on to me.

Can a relay stick ? I'm not sure.

Ah, hang on a minute, just a thought, my brake lights on my cabby will work without the key in the ignition, obviously me pressing the pedal, so, there should be some sort of plug by your brake light that makes a connection when you press the middle pedal and your lights come on, that might be your problem so have a look at that first.
