Tuesday, March 18th, 2008
Bleeding Shimano Disk Brakes
Today we are going to take a look at bringing some life back into your Shimano disk brakes. Chances are your bike has been sitting in the garage over the winter neglected and waiting for a spring tune up. Now is the perfect time to break out your toolbox and start on some maintenance. It is recommended you bleed your brakes at least once a year to flush out any foreign materials that might have worked themselves into the system. Maybe some air is trapped in the lines causing your brakes to feeling spongy. A proper bleed will having them feeling solid again.
With this guide, you’ll learn to bleed your brakes and clean up your brake pads and rotor to remove any contaminates that might have accumulated from oil/brake fluid, road grime, or glazing. After this your brakes will feel much better and will be ready for some serious vertical!
First gather the tools we’ll need to bleed the brakes:
- A bottle of mineral oil
- 7mm wrench
- Philips screw driver
- 10mm Allen key
- 4mm Allen key
- A small section of tubing
- Plastic bag with elastic band
Shimano recommends you only use their official mineral oil when bleeding their brakes. Alternatively a cheaper and slightly better option is Pentosin CHF 7.1 – it’s the same stuff that is used in most European cars such as BMW. You can probably find it at places like NAPA or Lordco. Do NOT use mineral oil that is available in pharmacies. That stuff is sold as a laxative not brake fluid. It is much heavier of a weight then the Shimano or Pentosin at around 30 to 50wt and will lead to slow and sluggish performance of your brakes. If you bleed brakes a lot I recommend using the Pentosin as it is around 25 times cheaper then Shimano. Also, consider using a dedicated pop can instead of a plastic bag to catch the brake fluid to help save the environment. An old spoke can be used to hook the can by its pull tab onto your stem or shifter cables.
Now place your bike in the stand and rotate it so the line you are about to bleed first runs as close to vertical as you can get it without being inconvenient. Next remove the wheels and brake pads, being careful not to touch the braking surface of the pads if your hands are dirty. Set the pads and wheels off to the side well away from any potential spills. Place a clean 10mm Allen key in the caliper where the pads used to be to prevent the pistons from being pushed out. Using the 4mm Allen key, loosen your brake lever and rotate it so the reservoir is parallel to the ground and temporarily retighten it so it won’t move. Using the Phillips screw driver, remove the top cover on the reservoir and remove the diaphragm.
Now place the 7mm wrench on the bleed nipple and attach the piece of tubing with the plastic bag (or can) on the end to the bleed nipple to catch the fluid that will be removed from the system. Your setup should look something like this:
Take your bottle of mineral oil and top off the fluid in the reservoir. Now open the bleed nipple a 1/8th of a turn and squeeze the brake lever. You should see fluid start to come out and fill the tube attached to the bleed nipple. Do not release the brake lever until you close the bleed nipple again. Doing so will draw air back into the system.
Close the bleed nipple and then release the brake lever. You should see the level of fluid in the reservoir drop after releasing the brake lever. You can usually repeat this process twice before having to top off the reservoir again. If you squeeze the lever without the reservoir having sufficient fluid in it you will again draw air into the system. By doing this multiple times you draw in the new fluid which will purge the air out from the lines. Keep repeating this process until no more air comes out of the caliper or you see the new fluid if you are doing a full bleed. While doing this you can carefully tap the brake lever and caliper to help move the air bubbles out from the system. You can also keep the bleed nipple closed and squeeze the brake lever repeatedly to help remove air from within the brake lever / reservoir itself.
Now squeeze the brake lever while rapidly opening and closing the bleed nipple about four times. Keep the brake lever squeezed as a final test to check for any tiny air bubbles that might escape. If none come out and the brake lever is stiff when pulled bleeding is complete. If the lever is still soft repeat the previous steps until the lever is firm. Top off the reservoir with mineral oil to the point of overflowing and replace the diaphragm and the top cover. Rotate your brake lever back into position and double check that the bleed nipple is tightened sufficiently. Wipe off your brake lever and caliper of any spilled fluid with a rag that has been dabbed in isopropyl alcohol.
That pretty much sums up bleeding a Shimano disk brake. Before you install your pads and rotor again before to check the pad wear and replace if they are looking worn down. If your pads and rotor have been contaminated with oil, or if your pads look glazed and don’t have the stopping power they used to, take the following steps:
Recovering lost performance:
If your pads have oil on them there are 4 common options on how to remove it: isopropyl alcohol, automotive disk brake cleaner, acetone, and extreme heat. Shimano recommends you only use isopropyl alcohol on their disk brakes to clean them. Disk brake cleaner and acetone can eat the material that binds the pads to the backing and can damage the seals of your caliper and other plastics. If you do use disk brake cleaner remove the pads from the caliper and the rotor from the hub to be sure no cleaner lands on anything important, I wouldn’t want that stuff working its way inside my hubs or splashing on my carbon fiber derailleur.
For any of these methods be sure to use a clean rag that hasn’t come in contact with grease or paper towel. For isopropyl alcohol dab a clean rag in the alcohol and wipe down your rotor and pads being sure to get every area. For disk brake cleaner, spray the cleaner on to the surface you wish to clean, wipe off with a rag and rinse with water. If these methods can’t bring back performance you can try burning off the offending oil by using a blow torch, BBQ, or an oven set at 400 degrees for 20 minutes. Most likely if you’re putting your pads in the oven its probably just best to buy a new set of pads.
If your pads are glazed because of overheating or improper breaking in technique, place the pads on a flat surface and rub in a circular motion with sand paper or a Scotch-Brite pad to remove the glaze. Give them a rub down with a rag soaked in isopropyl alcohol to remove any particles left over from the sanding.
After cleaning / sanding your pads and rotors they will need to be broken in again. Do not ride to the top of the highest high and scream down yanking on the brakes at the last moment to break them in. This sudden increase in temperature causes the pads to glaze over resulting in poor performance. To properly break in your pads and rotor, select a low gear and slowly pedal around your block with the brakes lightly applied. You should start to feel the pads get more grip as you continue riding. As that happens apply more pressure to the brakes and pedal harder to the point where you can feel you have reached maximum braking power. Once the pads have bedded-in, feel free to rip down that giant hill!
