Sram brake bleeding (DOT)

Purge de freins Sram (DOT)

Since the advent of hydraulic disc brakes in mountain biking, mechanical brakes have taken a back seat. More powerful and offering more modulation, hydraulic disc brakes are the obvious choice for descending mountainside slopes at full speed.
The system consists, very simply, of a master cylinder at the lever which, when activated, creates a high pressure zone in the hydraulic fluid line. As the fluid inside the line is incompressible, the high pressure is transmitted to the caliper where pistons (2 or 4 depending on the brake model) advance to balance the system, and in doing so, push on platelets. These pads come into contact with the disc and slow it down by
transforming kinetic energy into heat using friction. The principle is the same as with all other brakes, only the way of making contact between the pads and the disc changes.

However, hydraulic brakes require special attention and a (barely) more complicated maintenance procedure. Indeed, in order to guarantee optimal performance and lifespan, it is recommended to bleed the hydraulic fluid from the brake system at least once a year, depending on the level of use. In addition to ensuring fluid cleanliness, bleeding eliminates air bubbles that may have entered the system and are detrimental to braking performance. If your brakes are spongier than usual, if the contact point (bite point) is no longer the same as before, or if the lever needs "pumping" before it is firm to the touch, it It’s high time for a purge.

This tutorial is aimed at SRAM brake users who use DOT 5.1 synthetic oil and is heavily inspired by the official SRAM procedure available here: UM%20-%20DOT%20Fluid%20MTB%20Disc%20Brake%20Hose%20Shortening%20and%20Bleed%20Manual#hashItem=hose-shortening



*Be careful, DOT 5.1 oil is very corrosive and harmful to your health, wear safety equipment and make sure to wipe up every drop that falls on the shifter, caliper or any other part of your bike*

1: Preparing the syringes:

  • Install the tip with threads on the first syringe (the one for the controller).
  • Fill this syringe with approximately 15 mL of DOT 5.1 or DOT 4 liquid.

Install the Bleeding Edge tip or tip with threads onto the second syringe (the one for the caliper).

  • Fill this syringe with approximately 5 mL of DOT 5.1 or DOT 4 liquid.
  • Be sure to remove any air bubbles by pointing the syringes up and gently pressing down on the plunger until all bubbles are released. Close the syringes using the clamp on the tube.

*If the DOT 5.1 fluid is older or the bottle is already opened, air may be dissolved in the fluid. To ensure it is properly removed, create a low pressure zone by pulling on the plunger. Small bubbles should grow and rise to the surface. Repeat until there are almost no more bubbles. Reopen the syringe and eliminate the new bubble formed.

2: Preparing the controller

  • For brakes with the contact point adjuster, turn the dial in the opposite direction of the arrow until it locks.
  • Adjust the reach of the lever until the tip of the lever is 75-80mm from the center of the handlebars.
  • Slide the rubber boot off the lever tip to prevent hydraulic fluid from getting underneath and corroding the nut.
  • Open the purge port using a T10 Torx wrench. Screw syringe 1 (the one with 15 mL of liquid) into the purge port.

3: Preparing the caliper:

  • Remove the wheel.
  • Remove the screw that holds the pads, then remove the pads. Place them in a safe place where there is no chance of contaminating them. It is recommended to clean them with a clean towel and rubbing alcohol or brake cleaner.
  • Measure the thickness of the pad. If the total thickness of the board (including the backing) is less than 3mm, it is time to replace them.
  • Measure the thickness of the disc in several places. If you get a measurement lower than 1.55mm for Centerline discs and 1.7mm for HS2 discs, it is recommended to replace the disc.
  • Clean the caliper with a cloth or cotton swab and soap or isopropyl alcohol.
  • Place a 4mm hex wrench between the pistons and squeeze the brake lever until all pistons contact the wrench.
  • Clean the pistons well with isopropyl alcohol and a cotton swab.
  • Lubricate the edges of the pistons with a cotton swab dipped in DOT 5.1 or 4 fluid.
  • Gently push the pistons back completely using a plastic tire spoon. Make sure the syringe at the handle is fully open; pushing the pistons back without opening the system risks damaging the seals or the diaphragm.
  • Clean the caliper again with isopropyl alcohol.
  • Place the correct bleed block for your brake model into the caliper and secure it in place using the pad screw.
  • Open the bleed port at the caliper. If you have a brake model that uses a threaded bleeder port, use a T10 Torx wrench, then screw in your second syringe. If you have a brake model that uses a Bleeding Edge bleed port, gently remove the rubber tip and open the screw inside a quarter turn using a 4mm hex wrench, then close it gently. Insert the Bleeding Edge Tip Syringe until you hear a click. Open the port by turning the red dial all the way. Open the syringe by opening the clamp on the tube.
4: Purging the system:
  • Make sure both syringes are open.
  • Holding the controller syringe vertically, gently push the liquid from the top to the bottom, until only 3-5 mL remains in the controller syringe. If the fluid coming out of the caliper into the bottom syringe is dark or discolored, remove the syringe, dispose of the fluid, and refill the syringe.
  • Holding the syringe in the yoke vertically, gently pull the syringe plunger at the handle to return the liquid. Stop when there is only 3-5 mL left in the caliper syringe. Repeat these steps until there are no more bubbles coming out of the controller or caliper.
  • Disconnect the lever from the yoke.
    • For brakes that use the Bleeding Edge tip, turn the dial clockwise to close the system, then pull firmly on the dial to disconnect it from the brake. Wipe up any spilled liquid with a cloth and isopropyl alcohol and replace the rubber stopper.
    • For brakes that use a threaded tip, unscrew the syringe from the bleed port and close it using a T10 Torx wrench. Wipe up any spilled liquid with a cloth and isopropyl alcohol.
    • Once the system is closed to the yoke, firmly press the joystick lever and release. Repeat the operation ten times.
    • Pull firmly on the syringe plunger at the lever to create a low pressure zone and draw any remaining air bubbles out of the brake system. Release and push the plunger. Repeat the process until there are no more bubbles coming up into the controller.
    • Restore normal pressure to the system by pushing on the plunger.

    *The official Sram procedure ends here, removing the syringe from the controller and closing the purge port on the controller using a T10 Torx wrench. However, in order to maximize the effectiveness of the purge, our mechanics recommend a final step.

    • Remove the bleeder block and reinstall the pads in the caliper.
    • Place a pad spacer between the pads.
    • Repeat the process of creating a low pressure zone and releasing in the syringe with the handle until there are no more air bubbles rising into the syringe. Between each repetition, firmly squeeze the brake lever about ten times.
    • Once all air bubbles are removed, gently depress the syringe plunger to restore normal pressure to the system.
    • Unscrew the syringe and close the purge port using a T10 Torx wrench. Wipe up any spilled liquid using a cloth and isopropyl alcohol.
    • Remove the pad spacer .
    • Reinstall the wheel.
    • Center the rotor between the pads using the caliper adjustment screws.
    • Firmly squeeze the brake lever a few times to ensure proper advancement of all pistons. The controller should feel firm after these few presses. If it is not, there is still air in the system and the procedure must be repeated. Otherwise, congratulations, your brake is bled. Good bike!