Be absolutely sure there are no leaks, check the seals on the caliper pistons, they are easy to replace. Once you have established the fluid is not leaking out anywhere it is air in the line.
Take your caliper off and hang it from the ceiling overnight, I know this sounds odd(and hard), but air goes up.
This is a two people job,keep the caliper above the master cylinder( make sure youve put someting in between the brake pads.Now bleed system, hopefully you will see air escaping from the bleed nipple.
We had a similar problem and it was just a small amount of air, I could not believe it.
BTW do you have hand gaurds??
Is your brake lever able to extend fully, IE it doesn't get hung up on the back of the barkbuster? This is often overlooked as well and will give similar brake problems.
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only dot 4,3,5.1