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Master Cylinder

WARNING: This page is about a different car, the 2023 Subaru Ascent. However, it is still accessible from the selected car via links, so may be relevant.
CAUTION:
  • During the operation, disconnect the ground terminal from the battery sensor to prevent the electric brake booster from starting.
  • Do not let brake fluid come into contact with the painted surface of the vehicle body. Wash away with water immediately and wipe off if it is spilled by accident.
  • Do not mix different kinds of brake fluid. Mixing different brands of fluid, or mixing different grades of fluid (even if they have the same brand name), may lead to severe accidents due to reduced brake fluid performance.
  • When refilling brake fluid, do not allow dirt or dust to get into the reservoir tank.
  • Always use new SUBARU genuine brake fluid when replacing or refilling the fluid. Do not reuse drained brake fluid.
  • For convenience and safety, perform the work with 2 people.
  • During the operation, keep the reservoir tank filled with brake fluid at MIN level or higher to prevent entry of air.
  • After air bleeding operation, wipe off fluid around each bleeder screw and the reservoir tank.
NOTE:
  • When the master cylinder assembly is replaced or the reservoir tank is empty, bleed the brake master cylinder.
  • If air bleeding operation of the master cylinder assembly is not necessary, omit the following procedures, and perform air bleeding of the brake line. Ref. to BRAKE>AIR BLEEDING>PROCEDURE>BRAKE LINE .
  1. Disconnect the ground terminal from battery sensor. Ref. to REPAIR CONTENTS>NOTE>BATTERY .
  2. Install the pipe assembly front ABS to the master cylinder assembly.
    1. Screw and tighten the flare nut (b) so that the end of the nut contacts the back side of the pipe assembly front ABS flare, while pressing the pipe assembly front ABS (a) toward the master cylinder assembly side.
      CAUTION: Be careful not to make scratches or other damage to the inside surface of the pipe assembly front ABS flare.
      G13199763Courtesy of SUBARU OF AMERICA, INC.

      Preparation tool: 

      Flare nut wrench

      Crowfoot wrench

      NOTE: If the work can be performed with the front and back sides reversed, it is also possible to use a 6-point crowfoot wrench.

      Tightening torque: 

    Calculation formula
    T = 19 N.m (1.9 kgf-m, 14.0 ft-lb) xL1/(L1 + L2)
    T: Reading of the torque wrench
    L1: Effective length of the torque wrench
    L2: Effective length of the crowfoot wrench
    NOTE: If the effective length of the tool used is unknown, consult the manufacturer of the tool.
    G16349749Courtesy of SUBARU OF AMERICA, INC.
    (a) Effective length of the crowfoot wrench (L2)
    (b) Effective length of the torque wrench (L1)
    (c) Center of the open end of crowfoot wrench
    (d) Center of drive square of the torque wrench
    (e) Center of the position where a force is applied by hand
  3. Fill the reservoir tank of the master cylinder assembly with brake fluid.
    NOTE:
    • Replenish brake fluid to the MAX level.
    • During the operation, keep the reservoir tank filled with brake fluid at MIN level or higher to prevent entry of air.
  4. Attach a transparent vinyl tube to the bleeder screw of the right front caliper body that is the closest from the hydraulic unit assembly VDC, and the other end of the vinyl tube to a collection container.
  5. Loosen the bleeder screw, depress the brake pedal slowly and hold it.
  6. Tighten the bleeder screw, and release the brake pedal quickly.
  7. Repeat steps  5 and  6 until there are no more air bubbles in the brake fluid in the vinyl tube.
    NOTE: Air bubbles are removed after repeating the procedures six to seven times.
  8. Attach a transparent vinyl tube to the bleeder screw of the left front caliper body, and the other end of the vinyl tube to a brake fluid collection container.
  9. Repeat steps  5 and  6 until there are no more air bubbles in the brake fluid in the vinyl tube.
    NOTE:
    • Air bubbles are removed after repeating the procedures seven to eight times.
    • With the procedures so far, bleed is completed for the air that entered in the master cylinder, front brake piping hose passages and front caliper. Next, perform the following procedures to bleed air that entered in the rear brake piping hose passages and rear caliper from the hydraulic control unit.
  10. Attach a transparent vinyl tube to the bleeder screw of the left rear caliper body, and the other end of the vinyl tube to a brake fluid collection container.
  11. Loosen the bleeder screw, and repeat the full stroke depressing operation of the brake pedal slowly, until the brake fluid in the transparent vinyl tube has no more air bubbles, then tighten the bleeder screw.
    NOTE: By repeating the procedures 15 times or more, air that entered in the rear brake pipe passages from the hydraulic unit assembly VDC reaches the bleeder screw of the left rear caliper body assembly and is discharged.
  12. Attach a transparent vinyl tube to the bleeder screw of the right rear caliper body, and the other end of the vinyl tube to a brake fluid collection container.
  13. Perform step  11).
    NOTE:
    • By repeating the procedures 10 times or more, air that entered in the rear brake pipe passages from the hydraulic unit assembly VDC reaches the bleeder screw of the right rear caliper body and is discharged.
    • With the procedures so far, bleed is completed for the air that entered in the rear brake piping hose passages and rear caliper from the hydraulic control unit. Lastly, perform the following procedures to bleed air that may remain in areas where the brake fluid flow stands.
  14. Attach a transparent vinyl tube to the bleeder screw of the right front caliper body, and the other end of the vinyl tube to a brake fluid collection container.
  15. After repeating firmly depressing of the brake pedal quickly 5 to 6 times, depress and hold the pedal.
  16. Loosen the bleeder screw to drain brake fluid. When the brake pedal reaches the full stroke position, tighten the bleeder screw quickly, and release the brake pedal.
  17. Repeat steps  15 and  16 until there are no more air bubbles in the transparent vinyl tube.
    NOTE: Repeat the procedures approximately four to five times.
  18. Repeat steps  15 to  17 above for each brake caliper.
    NOTE: Bleed air in the order starting from the front RH → front LH → rear LH → rear RH.
  19. Tighten the bleeder screw to the specified torque.

    Tightening torque: 

    17 N.m (1.7 kgf-m, 12.5 ft-lb)

  20. Install the bleeder cap to each bleeder screw.
  21. Connect the ground terminal to battery sensor. Ref. to REPAIR CONTENTS>NOTE>BATTERY .
  22. Check that the electric brake booster operates normally.
  23. Check that there are no brake fluid leaks at the flare nut portion or in the entire brake system.
  24. After inspection, wipe off fluid around each bleeder screw and the reservoir tank.
  25. Perform a road test and ensure that the brakes operate normally.
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