[CT Jack]

It appears your brake switch might be the culprit. Your explanation of how everything works fine until you engage the brake switch says to me the brake switch has a high resistance connection. This condition is drawing down the voltage thereby preventing the flasher to work properly and dimming the lights. I would install a new brake switch. If you have a properly working voltage regulator, the directionals with the brake switch on should all work at idle. If your brake switch is okay then I would try to adjust the voltage regulator output at idle.