Mountain Bike Hydraulic Brake Bleeding

If you have hydraulic brakes fitted to your mountain bike, sooner or later you will need to bleed them to maintain their effectiveness.

Bleeding the brakes is the process of removing as much air from the hydraulic system as possible and replacing old brake fluid with new.

Bleeding of hydraulic brakes will become necessary if you break into the hydraulic system to, for example, renew or change the length of the hose or to clean out the pistons in the caliper if they are sticking.

More generally however, over time, the hydraulic brake fluid will become contaminated with moisture from the atmosphere and possibly air will find its way into the system.

DOT fluid is highly hygroscopic - that is it absorbs moisture from wherever it can find it. As the brake pistons are operated, air and even moisture can leak past the seals and into the system.

Generally symptoms of air in your hydraulic system are a spongy feel and lack of braking power. You could even find that the lever can be pulled all the way to the bar without the brakes working properly. Sometimes, pumping the brakes improves the braking but this is only temporary.

Hydraulic systems rely on the fact that fluids are generally pretty well incompressible. When you pull on the lever at the bars, all the force is transferred through the system to the caliper at the wheel, squeezing the brake pads on the disc to stop the bike.

If however there is air in the system, some of the force you apply to the lever will be used to compress the air, since gases ARE compressible, thus reducing the efficiency of the braking.

Moisture in the system will reduce the boiling point of the brake fluid (see wet and dry boiling points below). This will reduce the ability of your brakes under heavy use, for instance when you are descending a long steep down hill section. Moisture can also damage the system from the inside by corroding the metal.

DOT 4 and DOT 5.1 fluids are based upon Glycol Ether and are the most commonly used for hydraulic disc brakes on mountain bikes, including Avid, Formula and Hope. Be careful though; Shimano and Magura brake systems use a mineral oil based fluid which is NOT interchangeable with DOT ( US Department of Transportation) specification fluids. Seal materials are different and will cause brake failure if the wrong fluid is used.

ALWAYS USE THE BRAKE FLUID TYPE SPECIFIED BY YOUR BRAKE MANUFACTURER - FAILURE TO DO SO COULD DAMAGE YOUR BRAKES AND THENCE YOU, PERMANENTLY!

Note: DOT 5 fluid is silicon based, rare and generally only used in military vehicles. It should not be used in other DOT specified systems like Avid or Hope. It is NOT compatible despite the similar sounding specification!

Any brake is expected to stop you by converting forward motion, kinetic energy, into some other form of energy, mostly heat. When the heat cannot be dissipated by the disc, eventually the fluid can boil. When this happens, gas becomes present in the system and you get a reduced performance and the same symptoms that you get when there is air in the system (see above).

Any Avid hydraulic brake works with either DOT 4 or DOT 5.1. Avid’s own bleed kit comes supplied with DOT 5.1 fluid.

Red Dog Racing suggests that you use DOT 5.1 as this will carry on working for longer in heavy braking situations where high temperatures are likely and when you most need your brakes!

The DOT specifications for brake fluids are minimum standards that have to be met for a manufacturer to label it as complying to the DOT 4 or DOT 5.1 standard. The actual composition of the fluid varies across different suppliers but the minimum performance should not.

DOT 4 Dry Boiling point, 230°C min. Wet Boiling point, 155°C min.
DOT 5.1 Dry Boiling point, 260°C min. Wet Boiling point, 180°C min.

Dry boiling point is defined in the DOT standard as the boiling temperature of the fluid as new, with no moisture present. The Wet boiling point is specified as when there is 3.7% moisture by weight in the fluid, the typical situation after about a year of use.

Formula themselves very specifically specify DOT 4 and all their brakes are supplied with DOT 4 fluid. Although 4 and 5.1 fluids are compatible, I think their view is that you should stick with one or the other. If you want to use DOT 5.1, they recommend flushing out the DOT 4 when you bleed the brakes. If you would like DOT 5.1 supplied with your kit, let us know.