A brake cable on a bike doesn't need to pull as precisely as a gear cable but it needs to carry more power. It's easy to fit a brake cable.
Thread Into The Lever
Thread the brake cable into the lever on the bike. This is easiest if you take careful note of the path of the old cable before removing it. On a flat-bar brake you may be able to line up the slot in the lever with the barrel and the lock-ring of the barrel adjuster to slot the cable in and catch the nipple in the carrier. On a drop-bar lever it’s more likely that you’ll have to thread the cable into the leader. Check the manufacturers precise instructions for the lever. These are usually available on-line.
Measure The Outer brake cable
For the back brake on the bike you need the first run of brake cable to be long enough to allow the handlebars to turn fully without pulling on the cable. If you are replacing an old cable, and you’re sure the cable was the right length, and you don’t plan to move the position of the levers or the handlebars, you can match the new cable to old. New bikes are often set up with cables that are too long, so you may be able to replace the old cables with shorter ones which look tidier, transmit power better and create less turbulence in the air-flow.
Cut & Thread Outer brake Cable
Cut the outer-brake cable carefully, check there’s no tag of metal covering the entrance hole. If there is snip it off. Use a spike to re-open the plastice liner. Apply a very small amount of grease to the opening and thread the inner brake cable in. It will pull the grease into the outer brake cable.
Continue down the cable run setting up each section of outer brake cable, remembering to fit any necessary ferrules. The last run into brake needs to be long enough for the brake to move.
Thread the inner brake cable into the brake and screw any barrel adjusters on the cable run until they are almost closed. They may be at the exit of the levers or on the entry to the brake. If you leave a couple of millimetres of barrel screwed out you can make final adjustments to the cable tension in either direction once the cable is clamped.
Clamp The Inner brake cable
Clamp the inner-cable onto the brake. The clamp bolt needs to be tight enough to hold the cable in position but not so tight it flattens the cable as strands will break. You need to do this with the cable under tension and the brake held in the ‘ready’ position not wide open.
Depending on the model of brake holding the brake in the ready position while pulling the cable can be a quite tricky.
You can use a ‘fourth-hand’ tool that uses a scissor action to pull the cable and push the brake at the same time.
You can open the barrel-adjuster and lock the cable with the brake clamped ‘on’ rather than in the ‘ready’ position, then close the barrel adjuster to loosen the brake into the ‘off’ position.
You can use a toe-strap to hold the brake in the ‘ready’ position while you pull and tighten the cable.
You can clamp a hand-vice on the end of the cable to that gravity and the weight of the tool hold the brake cable in tension.
You can find a helper and use them as a third-hand.
Take Up Slack
If pulling the brake ‘on’ has released any slack loosen the clamp bolt, pull some slack through and reclamp the cable.