Brake bleeding isn’t a task you’ll regularly need to carry out. The occasional service and fluid change is necessary but otherwise, as long as the system is kept free from air, you can forget about fettling and just go and ride. However, if air has managed to get into the system you’ll need to get the tools out.
The bleeding procedure doesn’t vary between the two fluid types – the aim is the same, though the method may differ from model to model. Both types of fluid will ’wear out’ over time; DOT should last a year or so if you’re doing a lot of heavy braking (ie riding or racing downhill) and up to four times that if you’re just hitting the trails once a week. Mineral oil doesn’t last as long and should be checked every six months or so; pop the cap off the reservoir and if the oil has lost its pinkish tinge or turned cloudy, it should be replaced.
Don’t be tempted to treat brake bleeding as a regular task. Unless there’s actually something wrong with the performance of your brake, fiddling with it isn’t going to be of any benefit. If you frequently find yourself with real reason to bleed your brake, ie. the lever starts to feel spongy very quickly, then you need to track down the fault that’s causing the problem. Disc brakes are very resilient and rarely ’just go wrong’ – so take a close look at your hoses to see if there are any nicks or kinks along their length. Even if the outer surface of the hose doesn’t appear to have a hole in it, it’s possible for a kink to pierce the inner nylon pipe which holds the brake fluid, which will then seep along the fibrous sheath which protects the inner pipe. Baffling until you find the flaw, and then very obvious indeed.
Trapped air bubbles make your brakes feel spongy
Similarly, if your piston seals have been damaged by dirt ingress or one of the pistons has even been cracked or chipped, air will get into the system easily and hinder your stopping. Pull the pads out and check for visible damage or an oil leak; if you find a problem then the brake needs to go back to the bike shop for repair.
Regardless of model or brand specifics, the purpose of brake bleeding is to remove all air from the system, leaving only brake fluid within the hose, calliper and lever. It’s a job which is made much easier if you have the correct bleed kit; in fact, you won’t actually be able to bleed certain brakes without their dedicated bleed kit as they require a special screw-in bleed nipple. Other brakes use simpler tools, but the theory behind them all is the same.
Arm yourself with plenty of rags, a can of spray disc braker cleaner and mechanics’ gloves before you start, too. Brake fluid, particularly DOT, is harmful to skin, paintwork and the kitchen floor so try not to splash it around and mop up any spillages immediately. If you manage to get fluid on your skin rinse it off immediately and, particularly in the case of DOT, consult the instructions on the bottle for any further action.
If your brake’s bleed nipple is simply opened by a spanner, then you don’t have to splash out on a dedicated bleed kit. Instead you can make one up yourself using plastic hose from a car repair or model shop that attaches snugly to the nipple and a plastic syringe from the chemist to top up the reservoir or force oil into the calliper, depending on how your system works. A small plastic bottle with a screwtop lid that you’ve punched a hose-size hole in is by far the easiest way to catch the surplus brake fluid, particularly if you’re using DOT which will melt the thin plastic bags you sometimes find in mineral oil bleed kits. Make sure that the bottle is never used for food purposes, though, and tape it to the fork leg or frame to avoid it pulling the hose off the bleed nipple as the weight of the surplus oil increases.
Be careful when loosening and tightening bleed nipples. Avoid backing them off so far that they start to leak oil around the thread; this is a sure-fire way to find yourself puzzling over why you’re still finding bubbles in the system after half an hour of work. Where oil can get out, air can get in.
Refer to the instruction manual for your own brake’s particular quirks; the examples that follow are just that, but they should give you an idea of how the job you’re trying to do should work.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES