Bleeding Hydraulic Brakes
To bleed a brake, refers to the process of replacing the fluid within the brake system. This is done periodically to maintain the quality of the brake system. By keeping on top of this, you can avoid expensive replacements, and enjoy a fresh feeling brake system. Over time, hydraulic brakes can become spongy, and ineffective in braking. This is a typical sign that brakes need bleeding. Once a system is fully bled (well), it will feel firm and will extremely responsive.
Avid and SRAM both hold a fairly wide range of disc brake models, which all work under very similar technologies. If unsure, please check the SRAM website. Therefore, the bleeding procedure is very similar between models. However, to bleed these brakes, you will need a specific Avid/SRAM bleed kit, which aren’t too expensive. The bleed kit is sold in two standards, the ‘bleed kit’, and the ‘professional bleed kit’. Either will get the job done, however the professional kit is much easier, and will last a lot longer. The following will show you a step-by-step guide to bleeding your brake system, like a professional!
Setting Up To Bleed
Before bleeding your system, you will need;
– Avid/SRAM bleed kit + Dot 5.1 fluid (hydraulic fluid)
– Allen key set
– Isopropyl alcohol (Or an oil free cleaner)
– A cloth or rag
– Tyre lever, or another tool of equivalent shape (to push back pistons)
– T10 Torx key (comes with bleed kit)
– Bleed block (comes with bleed kit)
– Pliers (depending on model)
– A bike stand (recommended)
– Rubber or Nitrile gloves (recommended)
– Safety glasses (recommended)
Once all of these items are set up, we can begin the brake bleed!
Bleed the Brakes
The following is laid out as a step-by-step guide with tips and tricks to better bleed your brakes;
Beginning with the bike mounted off the floor in a stand (preferably), begin by removing the bike wheel. This avoids any chance of contaminating the disc. Set wheel aside.
Next, remove the brake pads from the brake caliper. This avoids any chance of contaminating the pads. This can be achieved by taking a small Allen key (can vary in size) and unscrewing the small pad retainer pin, located on the upside of the outside of the caliper. Alternatively, some older Avid models use a pin, which is bent to hold the pads in place. If so, take a pair of pliers, and straighten the pin, so it can slip out of the caliper. Proceed to remove the pads by grabbing the small brake pad end which houses the pin. Be sure to avoid touching the brake pad’s braking surface in order to avoid contamination. Set pads in a dry place away from the bike.
Take a small tyre lever, or similarly shaped tool (flat head screw driver works well too), and push down on the caliper pistons to compress them. Immediately place the bleed block which fits the caliper in between the caliper pistons, located in the caliper body.
For a more thorough bleed, remove the brake caliper from the bike, and let it hang. This allows a straight passage from the caliper to the lever, minimizing chance of air being trapped. While this step is not compulsory, it should deliver longer lasting and better results.
Now put on safety glasses, and gloves to avoid any fluids coming in contact with your eyes, or skin. Next, attach nozzles to the ends of the syringes, provided in the bleed kit. Take the first syringe and fill it one half full, with dot 5.1 brake fluid. Take the second syringe and fill it one fifth full.
To remove any gasses from the hydraulic brake fluid, hold the syringe with the nozzle facing up. Clamp the nozzle with the clamp provided, and begin to pull on the end of the plunger (reverse of pushing fluid out), to create a vacuum (don’t pull too far as seal will be broken. Only pull so there is some tension). You should see small bubbles appear around the outside of the fluid. Begin flicking the sides of the syringe, to release the bubbles, and allow them to float to the top of the syringe. Then release the tension off the plunger, un-clip the clamp, and press on the syringe plunger to remove the air from the inside of the syringe. Repeat this whole step until bubbles cease to form. Once gasses are removed, clamp both syringes and place aside.
Tip: Use an old rag or cloth to hold at the tip of the nozzle when removing air from the syringe, to avoid dropping fluid on the ground or your bike.
Before bleeding brakes, check the contact adjustment knob, and ensure it is rotated away from the arrow (so the lever pulls in closer). This allows the fluid to bleed the whole lever, more thoroughly.
Now that the syringes are prepared, look down at the brake caliper. On the caliper is a small screw, which will fit a T10 Torx key. Insert key and unscrew bleed screw counter clockwise.
Tip: Make sure key is fully inserted before turning, as these screws aren’t easily replaced if stripped.
Place bleed screw to the side.
Take the half full syringe (ensuring it is still clamped shut), and begin screwing the gold end of the nozzle into the brake caliper bleed port, which the bleed screw was just removed from.
Now go up to the lever and remove the bleed screw.
Note: The bleed screw can sometimes be hidden under the rubber lever end.
Take the ‘one fifth full’ syringe, and begin screwing the gold end of the nozzle into the brake caliper bleed port, which the bleed screw was just removed from.
Now that the system is set, it’s time to bleed it! Begin by un-clamping each syringe, first the bottom syringe, then the top. Hold both syringes upright, so any air bubbles float towards the back of the syringe body, and begin to squeeze the bottom syringe, while pulling upwards softly on the bottom of the ‘one fifth full’ top syringe. Allow the fluid to pass through from the bottom to the top.
Note: If the colour is darker than the new fluid, or is black, you may need to bleed it twice. While its not always necessary, it stops the new fluid lumping and becoming thick as quickly
Stop pushing fluid through the system once the bottom syringe becomes ‘one fifth full’, and when the top becomes half full.
Now clamp the top syringe, and pull the lever back to the handle bar. While holding the lever down, fasten it to the handle bar, with a strap, or even rubber bands.
Note: Ensure that the lever is right back, touching the handle bar to ensure the lever is fully open.
Moving down to the bottom syringe, begin pulling on the end of the plunger, creating a vacuum, to suck any old fluid or air which may still remain. Remember to not pull to hard, as you may break the seal. Once the vacuum has been held for a few seconds, push down firmly on the end of the syringe. Repeat this step 5-10 times, till air bubbles cease to exit the brakes.
Once bubbles have ceased, remove the strap (or rubber bands) which are holding down the lever. However, keep the lever compressed with your hand as the strap is removed. Now begin to slowly release the lever to its extended position, as you firmly apply force to the bottom syringe, filling up the system. Giving the bottom syringe one final push, clamp the nozzle, and remove the syringe and nozzle from the bleed port. Insert bleed screw back into the caliper, and wipe it dry.
Moving up to the top syringe, un-clamp the nozzle, and press down on the plunger. As you press down, very lightly squeeze the lever (only 1-2cm) and allow it to snap back quickly to its original position. This dislodges any air bubbles which may remain in the lever. Then begin to pull on the end of the syringe, creating a vacuum which pulls any air bubbles out. Repeat this whole step 5-10 times, until all bubbles cease to exit the lever.
Once bubbles have ceased, give the top syringe one last firm push, and clam the nozzle. Remove the syringe and nozzle, and insert bleed screw back into the lever.
Now that the system is fully bled, spray the lever, handlebars, forks, and caliper with a cleaner, or isopropyl alcohol. Wipe the system dry, then remove gloves, and any other safety equipment. Now clean your hands (to avoid contamination), and reinstall the pads, followed by the pad retainer pin.
Lastly, reinstall the wheel according to the manufacturers guidelines, and re-mount your caliper to your bike.
Tip: When mounting caliper, tighten both mounting bolts to the frame, leaving them just loose enough for there to be a little bit of play. Give your lever a few pumps to move the pistons closer in, then release the lever and softly tighten one mounting bolt. Move one end of the caliper back and forth slowly, while the wheel is spinning, to see which position the brakes rub less. Once found, tighten the loose side softly, and loosen the other. repeat until wheel rotates freely without rubbing at all. Once achieved, tighten the mounting bolts very firmly to stop any potential movement.
Give your system a quick check, by rolling along on it, and pulling the lever a few times. Don’t forget you can adjust how the lever feels, by adjusting it at the reach adjustment knob.
Congratulations! You have just successfully bled your Avid/SRAM disc brakes! If anything went wrong, please refer to the following paragraph on potential issues, and their diagnosis.
Why wont the fluid move from the bottom caliper, into the lever?
Generally this is because of two reasons. Number 1, the contact adjustment knob hasn’t been rotated. Or number 2, there is a blockage in the system. The most common reason Avid brakes won’t bleed properly is because they might have been subject to some impact, or they haven’t been used in a while.
If the brakes haven’t been used in a while, the system just needs flushing. Dot fluid is an extremely absorbent fluid, which over time, when not used, becomes lumpy and thickens. The way to fix this is by completely emptying the system. Take a small adjustable wrench, and unscrew the hose from the caliper, and from the lever. Try attaching the syringe to the caliper, and to the lever, and see if air will pass through them without the hose attached. If not (generally the lever), then something is either thickened and blocking one of them, or the system has been knocked, and something is broken or out of place in the lever. Lastly, try blowing air (either with a pump, or an air compressor) through the hose, while it is detached. this should free up any thickened Dot fluid from the hose.
The brakes feel fine, but why wont they wont lock up?
This can be from a range of things, but the two most common are; a contaminated set of pads, or a glazed disc. This generally means some fluid from the brakes, or another grease/degreaser has been in contact with one of these. If they make a squealing sound, that’s generally a dead giveaway. You’ll either need new pads or a new rotor (disc). However, I recommend replacing both, as if you have a glazed disc, it may rub off on your new pads, and vice versa.
Want to learn to fix other parts? Learn which tools you need at our “top 10 tools for cyclists’ article!
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