How to bleed hydraulic brakes on a mountain bike

October 03 2010

cycling

Yesterday I changed the tyres on my bike from hybrid to mountain bike in anticipation of the days ride, unfortunately the rain didn't clear so I cleaned my bike instead. Once the front wheel was back in I did a systems check and noticed the front brakes no longer worked. Ok they did, but only at about 5% effective. Time to do my first brake bleed, which I put off until Sunday (today).

The manual that came with my bike said to remove the calipers and let them hang vertically and attach a tube with a bag on the end to the fluild nipple on the calliper. First step was to procure me some mineral oil, which is the fluid used in the brake system. First step of step one was finding out where I can get mineral oil. Turns out mineral oil is also baby oil (99.95% mineral oil, 0.05% fragrance), so I ducked down to my local Coles and grabbed me the cheapest I could find: 500ml Coles brand Baby Oil for the princely sum of $2.29; this stacked up favourably to the $5.49 for 250ml of the Johnson variety and $23.50 for 1L official Shimano brake oil (100% mineral oil).

Baby oil acquired and urge to smother myself in it and roll around denied, I removed the calliper as per the instructions. I didn't have the required pipe and bag so I just let it hang and drip onto a rag on the ground. Worth a shot and I couldn't be bothered getting back in the car and driving an hour to chase down 10cm of 5ml clear plastic pipe. I followed the instructions for a while but failed miserably.

Then I flashed back to years spent helping my old man work on cars and my time spent helping him bleed brakes. I think I may have even done it for myself and mates a few times in my Uni days. For a car, one person puts pressure on the brake pedal while the other tightens and loosens the nipple on the calliper at the right time. Applying those principals to my bike I figured I should be able to do it myself. Full of confidence I put the callipers back over the discs and affixed them to the bike.

Ignoring my failed steps here's how I successfully bleed and fixed my brakes:

	1. Make sure the bike is horizontal and stable. Being cheap, I secured the bike on my car rack. 2. Remove the top of the oil housing, including the membrane inside.
	3. Remove the cap cover the nipple on each calliper (looks the same on the back brakes). Don't loosen it yet
	4. Take the top off the mineral oil/brake fluid reservoir. If this runs dry your brake line will suck air and you'll have to start all over again. Be sure to constantly monitor the fluid level. I found full two level pumps causing fluid to squirt from the nipple is enough to need to refill the reservoir. I used a kids medicine dispenser to ensure mineral oil didn't go everywhere. Let's just hope my kids don't get sick in the next few days! Remember: Mineral Oil is Mineral Oil. There's not need to spend $20 on Shimano brand.
	5. Now's where the real work is done. Pay attention to this because if you mess it up you'll have to start over: With one hand on the brake lever, use the other hand to open and close the nipple. You only need to turn it slightly, maybe, 1/8 to 1/5 of a turn. Open the nipple (loosen) and pull the brake lever all the way into the handlebar. You should see oil squirt out the bottom (some oil and some air). Before you release the brake lever, close the nipple. This is important otherwise releasing the lever will suck air buck up the line and you'll have wasted your time. Remembering to check the fluid level in the reservoir, continue this process until only oil is coming out of the nipple. Now you can start stage two. This is a refinement of the previous action. Instead of pulling the lever all the way to the handlebar, pull it in as far as you'd like the lever to be with the brakes fully applied. Remember to open the nipple before you start pulling the lever in, and remember to close the nipple before you release the lever. Repeat this action until your brakes are working as you'd desire. It should only take a few pumps