Q. I have replaced the original brake lines on my 2008 Honda CBR1000RR Fireblade as they were feeling 'baggy' on a long, fast ride, but I can't get the lever to firm up now. If I tie the lever in overnight it is fine for a short while, but soon goes soft.
A. As you've found out, brake lines have a four-year life at most as they start to expand excessively once the brake fluid has got hot.
To refill the system you need to have plenty of fluid to hand to keep topping up the master cylinder reservoir and in the bottle connected to the bleed nipples so that air can't get in when you release the lever.
Even with careful pumping and holding of the lever air can still be trapped in the system, it's worth tapping the calipers as you prime the system to help any air bubbles that are clinging to the inside of the caliper break off and float out.
Assuming you have done all this, there is another technique to employ which you can do at home alone. Start at the top of the system and just crack each union off one at a time, pumping the lever gently until fluid oozes from each joint, paying particular attention to any three-way union.
Obviously be very careful to retighten them correctly and avoid spilling fluid onto your bike.