FAQ

The term 'brake bleeding' refers to the process by which air bubbles are removed from the brake fluid used in hydraulic braking systems. The same process of bleeding air from the system is also used for purging the system of old brake fluid and replacing with new, therefore both procedures are usually performed at the same time.

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All hydraulic brakes are bled or purged for one of two reasons:

To remove air bubbles from the brake system, or
To replace the brake fluid with new.

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There is one classic sign of air inside any hydraulic mountain bike brake and that is excess lever travel - sometimes referred to as a 'spongy' or 'loose' brake lever.

If the lever has to be pulled a good distance before you feel the pads engage (or bite) the brake rotor then there is a good chance it is down to air inside the brake fluid.

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When you suspect air may be present in your brake system and your brake lever starts to feel spongy, it is time to bleed your brakes.

Some people advocate bleeding brakes every year or every two years as part of their service routine.

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Brake bleeding is a simple task which can be carried out by any half-competent cyclist. Once you've bled your first set of brakes you will wonder why you ever needed your local bike shop.

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The equipment needed to bleed hydraulic mountain bike brakes differs slightly from model to model but regardless of which brake you're trying to bleed you will usually need the following:

A bleed kit and brake fluid specific to your brake model. Basic tools including hex and torx keys and some protective equipment such as gloves and safety glasses to shield you from sometimes-corrosive brake fluid.

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A bleed block, or piston spacer as it is sometimes known, is an object designed to fit snugly between the pistons of your brake caliper (once in the reset position), and its job is to prevent them from moving outwards during the bleed process.

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The easiest way to reset the position of your caliper pistons is with the brake pads in situ.

Simply push a flat blade screwdriver in between the brake pads and twist. This will separate the brake pads and, in turn, push back the pistons to the reset position.

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The short answer is no. Emptying the brake lines of old fluid will only fill your brake with air - the very thing you are trying to remove.

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SRAM Bleeding Edge is a relatively new (2017+) technology from SRAM whereby they introduced a new bleed port design on their brake calipers.

The idea behind it was to make brake bleeding easier by using the bleed fitting itself to open and close the system during brake bleeding resulting in a cleaner operation.

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Air and water ingress into a braking system can happen following a breach of the system - following a crash, or during hydraulic hose shortening for example. Moisture can also enter the system via these means but it can also occur naturally as some brake fluids are hygroscopic, meaning they absorb water over time.

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Air in the system causes a breakdown in communication between the lever and the pistons. Input forces created by operating the brake lever can no longer be transmitted effectively as these forces are wasted as the air inside the system is compressed.

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If you're having difficulty resetting the position of your caliper pistons it is probably because your brake has been filled with too much brake fluid. Read on to find out how to fix it.

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Unfortunately it means that there is still some air trapped within the brake system. Read on to find out our tips on how to solve the problem.

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There are two types of brake fluid used in hydraulic bicycle bike brakes today:

DOT Fluid and;
Mineral Oil

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The type of brake fluid you should use in your hydraulic brake depends on your brake model. You will usually find the type of fluid your brake was designed to use printed somewhere on the lever assembly or reservoir top cap.

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We've covered this topic in some detail. There's no short answer so read on to learn more about DOT fluid vs. mineral oil brake fluids.

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The main difference between these two brake fluids is in their boiling points. They can be used interchangeably, as they are completely compatible with each other, but DOT 5.1 generally has a higher boiling point and is considered higher-performing.

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Once opened DOT fluid should be discarded after 12 months. Mineral oil will last for years as long as it is kept clean and free from contamination.

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The Dry Boiling Point of brake fluid refers to the boiling temperature of fresh, new brake fluid from an unopened container. Whereas the Wet Boiling Point is defined as the temperature DOT brake fluid will begin to boil after it has absorbed 3.7% water by volume.

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Degassing the brake fluid refers to the process of removing the tiny air bubbles trapped inside the brake fluid. By creating a vacuum inside your syringe these tiny air bubbles expand and rise towards the tip of the syringe for easy removal.

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50ml of brake fluid is enough to perform a bleed on one set of cycle brakes (front and rear).

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