How much do I need to bleed the brakes on a mountain bike?

Wait…what bleed the brakes on a mountain bike? That’s a thing? Yes, and it is something that sounds more terrifying than it actually is. Bleeding the brakes on a mountain bike isn’t easy but it is doable. If I can do it, you can do it.

Brake bleeding is the process by which air bubbles are removed from the brake fluid used in hydraulic braking systems. The process of bleeding air from the system is also used to purge the system of old brake fluid. Thus you can simultaneously bleed the brakes and replace the brake fluid.

The good news is that if you have a bike with hydraulic disc brakes the need to bleed the brakes is once every three to five years. If you find that your brakes need to be bleed more often, there is most likely something wrong with your brake system.

Types of Brakes

Nearly all mountain bikes now use disc brakes. There are two types of disc brakes:

  • Mechanical disc brakes: Mechanical disc brakes are the most popular type of disc brakes for the average mountain bike. Mechanical brakes work with cables, like the rim brakes.
  • Hydraulic disc brakes: Hydraulic disc brakes are a recent addition to mountain bike disc brakes. Instead of cables and their housings, the hydraulic system uses hoses, reservoirs and brake fluid to actuate the measurement of the disc brake.

Advantage of the hydraulic disc brakes:

The hydraulic system is closed off from the environment. So unlike the mechanical system that utilizes cables, the hydraulic system prevents dirt and debris from the off-road trail from entering the brake fluid or the hose and cylinder.

The hydraulic system offers more power and control when it comes to the actual braking process.

Disadvantages of the hydraulic disc brakes:

Hydraulic brake systems have to be professionally fitted.

The smallest air bubble in the system could cause the lever to lock. Removing the air is called “bleeding” and it must be done with precision.

But don’t worry you can bleed your brakes, with precision. The need to bleed the brakes is when air gets in the system and the lever locks.

How Do I know If I Have Air Inside My Brake?

You know your hydraulic brakes need bleeding if you have air inside your brake. The sign of air inside any hydraulic mountain bike brake is when you feel excess lever travel. If the lever has to be forcibly pulled before you feel the pads engage the brake rotor, then you may have air inside the brake fluid.

But don’t worry, you can remedy the problem by bleeding the brakes. The caveat is hydraulic brakes seldom need bleeding. The air getting inside the brake is a worst-case scenario. But if you have to bleed your brakes, follow these steps:

Steps to Bleeding Hydraulic Bicycle Brakes

First you will need some supplies: an 8mm wrench (preferably ring ended); a 26mm wrench or an adjustable wrench; clean rags; small length of tubing or a bleed kit; the correct brake fluid (some use DOT and some use mineral oil)

On brake fluid: some mountain bikes use DOT fluid and some use mineral oil. You should only use the fluid your brakes were designed to use. Do not fluctuate. Do not improvise. Intermingling the fluid will cause more problems than simple having air in your brakes.

Remove the wheel and brake pads. Make sure you do not touch the pad surface with your fingers as this will contaminate them.

  1. Hold the bike upright and stable. The nipple on the brake caliper should be pointing upwards. Adjust the brake lever clamp so that it is horizontal to the ground. Put paper on the ground underneath the lever and brake to catch excess fluid.
  2. Undo the large polygon nut (master cylinder cap) on the brake lever with a 26mm wrench.
  3. Put the 8mm wrench over to bleed nipple.
  4. Attach the tubing to the nipple and attach a container or rag to catch the fluid.
  5. Now for the bleeding
  6. Top up the lever with the appropriate fluid.
  7. Undo the nipple a quarter turn and slowly pull the brake lever in and hold it
  8. Tighten the nipple up
  9. Release the brake lever
  10. Undo the nipple a quarter turn and slowly pull the brake lever in and hold it
  11. Tighten the nipple up
  12. Release the brake lever
  13. Undo the nipple a quarter turn and slowly pull the brake lever in and hold it
  14. Tighten the nipple up
  15. Release the brake lever
  16. You got it…right
  17. You will see fluid coming up the hose. The fluid will have bubbles in it.

Repeat this process until no more bubbles come out. Make sure that the fluid in the reservoir does not drop too low. Keep topping it up.

Replacing the master cylinder cap is the next step. This is a bit tricky.

  1. Unscrew the reservoir piston from the master cylinder cap
  2. Top up the reservoir to the top and place a rag around it
  3. Gently push the reservoir piston into the master cylinder so that the O-ring is just covered
  4. Put the Master cylinder cap on the master cylinder and gently screw the silver adjuster counter clockwise
  5. When this process has brought the whole assembly down low enough to engage the cap and the threads on the lever, then press gently down on the cap and screw the cap clock wise by hand finishing with a 26mm wrench
  6. Screw the adjuster counter clockwise as far as it will comfortably go
  7. Inside the caliper you will notice the pistons slightly poking out
  8. Undo the nipple once more to release the fluid until they are flush with the inside of the caliper
  9. Clean up any fluid
  10. Replace the pads and wheel and screw the silver lever adjuster clock wise until you have the braking you want
  11. Check for leaks and test ride

It sounds like a lot but that is only because the process requires precision, which you are more than capable of doing. The good news is that according to Bicycles Stack Exchange, “unless you open the system, the oil stays good for up to seven years. The “regular bleeding” that every hydraulic brake need should be at most every three to five years, unless some problem happens.”

This simply means you will probably only have to bleed the brakes on your mountain bike once maybe twice. Although a mountain bike like a car’s life span should be viewed in terms of mileage, on average a good mountain bike will last a decade or longer. This is based on wear and tear and not your desire to trade up or down.

How often you will need to bleed the brakes on your mountain bike will also depend on the type of brakes you have. If you have the mechanical disc brakes, you will never need to bleed them. You will only need to replace the cables and their housings when there is a problem.

Those who have settled for a cheaper hydraulic disc brakes may find that they need to bleed their brakes more often. With a bleeding kit costing around $30 each time this could get expensive.

So the moral of the story is, if you are going to use the hydraulic disc brake system don’t skimp on the system. A good hydraulic disc brake system should have you worried about bleeding the brakes one or twice during the life span of the bike.

Mike Rausa

I'm a 42 year old married father of 3 that fell in love with mountain biking late in life. Mountain biking quickly became my go to fitness activity. I created this blog to help beginners to advanced riders with tips and strategies to improve your riding experience. More About Me...

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