>>1983244It's usually excessive cable tension, meaning the brake needs to be recabled.
In a pinch, you can unbolt the cable (don't pull it all the way out), and drip some oil or lubricant on either end of inner, and try and work it in a bit without moving the frayed part of the cable back into the housing.
If the brake caliper is stiff to open and close by hand, with no cable, it's not about loosening the bolt, you need to service the caliper, ie, take it completely apart, clean it thoroughly, and lightly grease all the mating parts and threads.
Bicycle components rarely need just adjustment when they're stiff or with play, they need service and then adjustment.
This is especially true for single pivot calipers like that. Servicing them is a regular job and it's quite easy to do.
In a pinch, you can spray penetrant/lubricant at the mating parts and try to work it in, but that's a bodge.
As for that bolt being stripped out, you shouldn't have tried to adjust it with that bolt. You can probably dissasemble the caliper from the back by just going for the nut on the other side, after removing the brake.
If you need purchase on that bolt head, you can try hammering in a slightly larger imperial hex key or something else which might deform in there and fit, or, you can cut a slot in the head with a dremel or a junk chisel, and use a screw driver in the slot.
This may be the right time to simply upgrade the brake caliper.
