If you have an older bike, chances are it has drum brakes. On the outside you'll have a covered hub at the center of the wheel with a short arm that's usually connected to a rod for the rear brake, or a cable for the front. Your dirt bike might have drum brakes on both wheels, or just one. The drum is fixed to the wheel and on the inside there are two brake pads. When you activate the brake, the arm is pulled, which moves a cam on the inside, which presses the brake pads against the inside curve of the drum. When you release the brake, a set of springs in the drum, and a spring on the brake mechanism, will pull everything back to the open position.
Disc brakes have a disc/disk (go figure!) that is fixed to the wheel. The disc is also called a rotor. At one point on the disk, you'll see a mechanism straddling the edge of the disc. That mechanism has two brake pads, one on each side of the disc. When you activate the brake, those two pads “pinch” the disc between them. Instead of using a mechanical linkage or cable, disc brakes use hydraulic pressure. Your dirt bike's brake lever presses against the hydraulic fluid in the master cylinder. That pressure is transferred to pistons that press the pads against the disc. Thanks to the science of hydraulics, a little bit of pressure on the lever results in a LOT of pressure at the piston.
One challenge with hydraulic brake systems is that the standard plastic/rubber hoses will either stretch and/or swell, and/or flex when they're under pressure. The result is what you'll hear dirt bike riders refer to as mushy brakes – it's another way of saying you're not getting the precise control you want from your brakes. More expensive dirt bikes will use braided steel hoses (and occasionally solid lines) to eliminate or reduce mush. These are also available as after-market products if you want to upgrade your dirt bike's stopping ability.
Drum brakes and disc brakes both use brake pads. The trick with brake pads is you want them to have enough friction to slow down the dirt bike, but not so much that the slightest touch sends you over the handlebars. You also want them as durable as possible, but less durable than the drum or rotor/disc – because pads are cheaper and easier to replace than rotors and drums. As a result, there are a variety of brake pads for your dirt bike because different companies use different compounds to try and find the perfect balance of durability and performance. In the end, it's all going to boil down to a matter of personal preference and, as a rule of thumb, you get what you pay for.
Drum Brake Maintenance
The good news about dirt bike brakes is that they tend to keep working without putting a lot of effort into them. The bad news is if and when they do stop working, it's never at a good time (with the possible exception of when very irritating people are riding). So, even if the brakes are working, it pays to check on them regularly and spot problems BEFORE they become catastrophic.
No brake is maintenance-free, but drum brakes come close. The biggest thing you'll want to do with these is to keep them adjusted properly – a simple cable or linkage adjustment. As well, keep cables and linkages well lubricated.
At least once a season, you should crack open your dirt bike drum brakes and inspect the innards. If it's full of goop, gunk, or crud (sorry about the technical terms) you'll want to clean it. The easiest way is with a can of brake cleaner. Find a patch of ground you don't care too much about, and spray the sucker until it's gunk free. If you're an environmentalist and uncomfortable with getting brake cleaner on the ground, get a friend to do it while you close your eyes. Just kidding. You can do it over a bucket, and dispose of the residue in a responsible manner (like feeding it to random cats).
Inspect the brake pads. If they're worn and shiny on only a small portion, it means the brake pad, or shoe, is not contacting properly. I've never run into this problem, but I've been told you can sand the center of the shoe for a better fit. NOTE: A LOT OF BRAKE PADS CONTAIN ASBESTOS OR OTHER CARCINOGENS – WEAR A MASK IF SANDING OR BLOWING OUT DUST, ETC.
Check for broken or missing springs on the inside. If you're replacing the pads, it pays to replace the springs at the same time.
Other than that, make sure the moving parts are lubricated. Use waterproof grease and use it sparingly – you obviously don't want any making its way onto the pads or drum.
When you re-install the brake, apply it gently but firmly while tightening the axle nut. That'll help keep everything centered.
If you've decided to inspect your drum brakes because they don't brake well when they get wet, don't bother. The fact is, drum brakes are basically just for show once you've gone through a creek. The best remedy while riding is to ride the brake slightly to speed up evaporation of the water.
DISC BRAKE MAINTENANCE
As I mentioned earlier, disc brakes rely on hydraulic fluid. Generally speaking, the fluid that came with the bike will be fine – however, if you're a hard core dirt bike rider or frequent racer, your fluid can get hot enough to affect performance. High performance fluids are available.
The only things that are really going to affect your hydraulics are air and moisture. If your braking is starting to feel spongy, it's probably time to bleed it to get rid of air bubbles, or flush it completely to get rid of old (moisture-laded) fluid.
There are almost as many ways to bleed the brakes on dirt bikes as there are dirt bikes. If you think I missed something, or have an improvement to add, I welcome your comments.
Start with a couple of feet of clear tubing. Most home centers carry it in all sorts of sizes and sell it by the foot or meter (buy a few extra feet... the stuff comes in handy). At the halfway point, tie an overhand knot, but keep it loose.
Find the bleeder screw cap (you have a manual, right?). It should have a rubber cover or some sort of cap on it. Remove the cap and it'll look like a short metal tube with a hex fitting at the base. Find the correct wrench and loosen the bleeder slightly to get it moving, then snug it back down.
Place one end of your tubing onto the bleeder and the other end into a handy dandy receptacle – the back end of a random cat for example.
Open the bleeder again but keep the wrench and your wrenching hand handy. Slowly and evenly apply the brake and you'll see fluid moving through the tube and into the cat or other receptacle. When the fluid stops, generally when you've bottomed out the brake lever – KEEP THE BRAKE APPLIED – and tighten up the bleeder. Then you just keep repeating.
Check the fluid level in the reservoir every couple of pumps. If it gets too low you'll just introduce more air into the system.
You can either go until you think you've removed any air bubbles, or until you think you've cycled through all the old fluid. For the cost, I suggest going for the flush.
The one challenge in the whole process is that air bubbles want to rise up in a fluid. So trying to get an air bubble out your front brake line – which is mostly vertical – can be a pain. If regular bleeding doesn't get the air bubble out you can try a couple of things. One, get the brake assembly higher than the reservoir (disassembly required). Two, take the caliper off the disc/rotor, remove the reservoir cover, gently push the pads apart. This should push the fluid towards the reservoir, and hopefully take the bubble with it.
That's it for basic maintenance. I hope to get something together on rebuilding the master cylinder but I need to track down a rider or two with more experience on the subject. If you're one of those riders, and a decent writer, I'm open to guest posters. This site isn't about me and what I know... it's about riding and what ALL of us can learn. Drop me a line.
Take care, ride hard!