GALION a606 brake bleeders

My attempt to bleed the brakes on our GALION a 606 has been thwarted. I tried every wrench from 10mm(too small) to 12mm(too big) and every wrench from 3/8 to 7/16, including 11/32 with no joy.
Can someone enlighten me on this? Does it require a special application wrench? I searched a bit and figure on saving the frustration and ask the wise sages here. Thank you in advance