Firstly and without malice, please post your signature so we-uns can know all the details.
You say the brights work in what we call flash-to-pass. Bingo, your bulbs, fuses, relays are ok. I think you'll find the headlight switch is going bad.
In my truck I can flash the high's with the headlights off just as you describe - it bypasses the dash mounted switch. Good luck, my friend!