Thursday, November 6, 2008

Less air, more braking

Shimano, Magura and Tektro, three popular XC brake manufacturers, have been using mineral oil as a hydraulic fluid for a number of years now. Though mineral oil is durable, non-toxic, and won't strip the paint off your frame it lacks one very convenient feature of DOT brake fluid (used by Avid, Hayes, Formula, and Hope). DOT brake fluid has the unique property that air dissolves in it under pressure. This limits the possibility for total failure due to air leaks in the system, and generally keeps your brakes feeling firm. Because mineral oil does not absorb air, any air that leaks into the brake line takes up space, and because it is easily compressed, it gives your brakes a mushy feel, maybe even to the point where you have almost no braking at all. This happened to me last week. It was lame.

Fortunately, there are some simple things that can be done to prevent and/or repair such failures in mineral-oil brakes. (note: this only applies to open system brakes. If your brake doesn't have an oil reservoir, it's not an open system)

First trick...

You can avoid getting air in your brake lines most of the time by making sure that your brake levers are compressed any time the bike gets turned such that the calipers or lines are above the reservoir. Compressing the levers isolates the reservoir from the rest of the brake system, so if there's any air in there, it can't get into the lines. I didn't to this last week when I turned my bike over to fix a flat, hence my problem.

Second trick...

If you DO get some air in the lines, You don't usually have to rebleed your brakes, you need only a pair of rubber bands and some of patience:
  1. Prop up your bike such that all of the brake lines have a continuously ascending path from the caliper to the lever. Depending on your hose routing, you may or may not have to lift the front of the bike higher than the back to make this work out.
  2. Pump up the brakes by repeatedly compressing the lever as far as it will go, releasing fully in between, until the brake pads are firmly against the rotor when the lever is compressed, then hold it in the compressed position with a rubber band.
  3. Use the handle of a screwdriver or similarly benign object to tap along the length of a brake line, starting at the caliper. This helps to free air bubbles so that they'll start floating to the top.
  4. Wait. I usually just let the bike sit overnight.
  5. Remove the rubber band, release the lever, and push the brake pads all the way out by pressing the rotor from side to side. If you have a workstand, you can take the wheels off and use a screwdriver to push the pads out.
  6. Now you have all the air in the reservoir. If you're lazy, or don't have more oil, you can just leave it there (SERIOUSLY NOT RECOMMENDED), but then you have to be especially careful about not letting it back into the lines (see first trick). What you really should do is move on to the next trick.
Third trick...

If you know you have air in your reservoir, or if your pads are really worn and aren't quite self-adjusting the way they should, (want to know why this happens? Email us and we'll explain. [Edit: someone did email me, and I explained here.] ) you can simply top off your reservoir with fluid as follows:
  1. Prop up your bike against a wall or in your workstand and adjust the lever so the reservoir cap is level with the floor.
  2. Push the pads all the way out
  3. Remove the cap and the membrane,
  4. If you're just trying to remove air from the reservoir, skip to 6.
  5. Making sure there is sufficient fluid in the reservoir, squeeze and release the brake lever to adjust the pads against the rotor.
  6. Fill the reservoir with fluid to the top
  7. Making sure there are no air bubbles present, replace the membrane and the cap. Use a rag to catch the fluid that spills out. Once you place the cap on the reservoir, keep pressure on it until you screw it down so air cannot enter.
Now you're air free, though it's worth mentioning, if you do step 5 above there will be too much fluid in the system when you replace your pads and they'll rub. If this occurs, find the bleed screw on the caliper and while pushing out the pads with a screwdriver, loosen the bleed screw until fluid can seep out. Push the pads as far out as they will go, and while maintaining positive pressure on the pads with the screwdriver (you don't want air getting sucked back in), tighten the bleed screw again.

...and that's one less trip to the bike shop.


Anonymous said...

4th trick: use DOT :)

Anonymous said...

to clarify, use a brake system that uses DOT.

Richard said...

WOW! This really works. I have Magura SL brakes and the mushy feel has been driving me crazy. I followed your steps and it got the air out of the lines. THANKS!