Kx's have weak brakes
1.Get a 270mm over size braking disc kit before anything else
2.I tried a cr master cylinder on kx's before with less spongy feeling
3.Get the best pads you can buy
4.don't bother with a aftermarked brake line they do little especially with newer kx's but you could try it with 2000 but i try the other 1,2,3 before this