Brake line is unwound can I cut part off? Putting together new bike and then brake line got frayed (just one piece) going pretty far back and cannot thread it now. Can I cut the one frayed piece or does that ruin the brake line functionality?

Thank you
If it's only one strand then you can safely cut it off.  The problem is that this leaves a jagged end that will hang when you try to thread it, so some very careful work is required to get the cut end bent tightly back against the remaining cable.  Often this is more trouble than it's worth.