Brake Master Cylinder

This procedure replaces the brake master cylinder on a 2023 Rivian EDV 500, including brake line removal, hydraulic system disconnection, and complete brake system bleeding.

Warnings

Tools required

Parts

• Brake master cylinder assembly × 1 — Use OEM Rivian specification
• Brake line copper washers/seals × 4 — Use OEM specification
• Master cylinder to booster gasket/seal × 1 — Often included with master cylinder

Fluids

• DOT 4 Brake Fluid — 2 qt

Preparation

1. Park vehicle on level surface and engage parking brake
2. Place wheel chocks behind rear wheels
3. Disconnect 12V auxiliary battery negative terminal and wait 5 minutes for system capacitors to discharge
4. Verify high voltage system is fully powered down (consult service manual for HV lockout procedures)
5. Remove brake fluid reservoir cap and use turkey baster or fluid pump to remove as much brake fluid as possible from reservoir
6. Place fluid catch container and absorbent material under master cylinder area
7. Cover fender and painted surfaces near master cylinder with protective cloths
8. Locate master cylinder on driver side firewall behind brake booster assembly
9. Have all tools, parts, and at least 2 quarts of fresh DOT 4 brake fluid ready before beginning

Procedure

1. 1

   Disconnect brake lines from master cylinder
   Using the appropriate size flare nut wrench (typically 10mm or 12mm), carefully loosen each brake line fitting at the master cylinder. There are typically 2-4 brake lines depending on configuration. Use a flare nut wrench to prevent rounding the fittings. Allow brake fluid to drain into catch container. Label or photograph brake line positions if needed for reassembly reference.
   ⚠Use only flare nut wrenches on brake line fittings. Standard open-end wrenches will round off the soft brass fittings, requiring brake line replacement.
2. 2

   Disconnect electrical connectors
   Locate and disconnect the brake fluid level sensor electrical connector at the reservoir. Press the locking tab and pull connector straight out. On Rivian EDV models, there may also be a pressure sensor connector on the master cylinder body - disconnect this as well if present. Note connector orientations for reinstallation.
   ℹ️Do not force electrical connectors. Locate the locking tab before pulling to avoid damaging connector pins.
3. 3

   Remove brake fluid reservoir (if separate)
   If the brake fluid reservoir is separately mounted, remove the reservoir mounting bracket bolts or clips. Gently pull reservoir from master cylinder, being careful not to damage the rubber grommets. If reservoir is integrated into master cylinder, skip this step.
4. 4

   Remove master cylinder mounting nuts
   Locate the two master cylinder mounting nuts that secure the master cylinder to the brake booster. These are typically 13mm nuts accessible from the driver side engine compartment. Remove both nuts completely and set aside. Support the master cylinder as you remove the second nut to prevent it from falling.
   ⚠Master cylinder may contain residual brake fluid. Keep catch container positioned to catch any drips during removal.
5. 5

   Remove master cylinder from brake booster
   Carefully pull the master cylinder straight away from the brake booster assembly. The pushrod from the brake booster will separate from the master cylinder piston. Inspect the pushrod condition and ensure the pushrod boot/seal remains in place on the booster. Remove and discard the old master cylinder gasket/seal from the booster mounting surface.
   ℹ️Do not allow brake fluid to contact the brake booster vacuum chamber or electronic components. Clean any spills immediately.
6. 6

   Prepare brake booster mounting surface
   Clean the brake booster mounting surface thoroughly with brake cleaner and lint-free rags. Inspect the pushrod for damage, wear, or corrosion. The pushrod should move smoothly and return freely. Verify the pushrod boot is intact and properly seated. Check that the booster mounting studs are clean and threads are undamaged.
7. 7

   Prepare new master cylinder
   Remove new master cylinder from packaging. Verify it matches the old unit exactly. Install new gasket/seal on the mounting face of the new master cylinder (or on the brake booster surface per manufacturer instructions). If master cylinder is not pre-filled, bench bleed the master cylinder according to manufacturer instructions before installation to remove air from the cylinder bores.
   ℹ️Bench bleeding the master cylinder before installation significantly reduces brake system bleeding time and helps ensure complete air removal.
8. 8

   Install new master cylinder
   Carefully align the new master cylinder with the brake booster pushrod, ensuring the pushrod enters the master cylinder piston bore correctly. Slide the master cylinder onto the mounting studs and press it flush against the brake booster. Thread both mounting nuts onto the studs by hand to ensure proper thread engagement.
   ⚠Ensure pushrod properly engages master cylinder piston. Misalignment can damage the piston or prevent proper brake operation.
   Torque spec

   Master Cylinder Nuts25 Nm (18 lb-ft)
9. 9

   Torque master cylinder mounting nuts
   Using a torque wrench, tighten both master cylinder mounting nuts in a cross pattern to 25.0 Nm (18.0 lb-ft). Verify the master cylinder is seated evenly against the brake booster with no gaps.
   Torque spec

   Master Cylinder Nuts25 Nm (18 lb-ft)
10. 10

    Install brake fluid reservoir
    If the reservoir was removed separately, install new rubber grommets (if provided with master cylinder), then press the reservoir firmly into the master cylinder body or install to mounting bracket. Secure any mounting hardware if applicable.
11. 11

    Connect brake lines to master cylinder
    Install new copper washers/seals on brake line fittings if provided. Thread each brake line fitting into the correct master cylinder port by hand first to ensure proper thread engagement. Once all lines are hand-threaded, use the flare nut wrench to tighten each fitting to specification. Refer to your brake line routing notes or photos to ensure correct positioning.
    ⚠️CRITICAL: Cross-threading brake line fittings will cause brake fluid leaks and brake system failure. Always start fittings by hand and ensure smooth threading before using wrench.
    Torque spec

    Brake Line Fittings15 Nm (11 lb-ft)
12. 12

    Torque brake line fittings
    Using a flare nut wrench and torque wrench, tighten each brake line fitting to 15.0 Nm (11.0 lb-ft). Do not overtighten as this can damage the soft brass fittings. After torquing, visually verify each fitting is properly seated with no gaps.
    Torque spec

    Brake Line Fittings15 Nm (11 lb-ft)
13. 13

    Reconnect electrical connectors
    Reconnect the brake fluid level sensor connector and any pressure sensor connectors removed earlier. Ensure each connector clicks fully into place and locking tabs engage properly.
14. 14

    Fill brake fluid reservoir
    Fill the brake fluid reservoir with fresh DOT 4 brake fluid from a sealed container to the MAX line. Keep the reservoir filled above the MIN line throughout the entire bleeding process. Monitor frequently and refill as needed.
    ⚠Never allow reservoir to run dry during bleeding process as this will introduce air into the system requiring restart of bleeding procedure.
15. 15

    Bleed brake system completely
    Perform complete brake system bleeding in the proper sequence. For Rivian EDV 500, typical sequence is: Right Rear, Left Rear, Right Front, Left Front. Use either two-person manual bleeding or vacuum bleeder method. At each wheel, attach bleeder hose to bleeder valve, open valve 1/4 turn, pump brake pedal or apply vacuum until fluid runs clear with no air bubbles, close valve, and move to next wheel. After bleeding all four wheels, use diagnostic scan tool to cycle ABS pump and valves (if equipped), then re-bleed all corners. Repeat until all air is removed and brake pedal is firm.
    ⚠️CRITICAL: Incomplete bleeding leaves air in the brake system, causing reduced braking performance or complete brake failure. The brake pedal must be firm with no sponginess before returning vehicle to service.

Reassembly

1. Ensure brake fluid reservoir is filled to MAX line with fresh DOT 4 fluid
2. Install reservoir cap securely
3. Clean any brake fluid spills from engine compartment and painted surfaces immediately with water
4. Reconnect 12V auxiliary battery negative terminal
5. Verify all brake lines are properly secured with no rubbing or interference
6. Check that all electrical connectors are fully seated and locked

Verification

• Before starting vehicle, pump brake pedal 20-30 times. Pedal should become firm and hold pressure. Soft or sinking pedal indicates air in system - repeat bleeding procedure
• With engine/vehicle powered on, verify brake warning light extinguishes and no brake system fault codes are present
• Inspect all brake line connections at master cylinder for leaks while assistant applies firm pressure to brake pedal
• Perform low-speed brake test in safe area (5-10 mph) to verify braking function before road test
• Perform road test in low-traffic area with multiple brake applications from various speeds, verifying firm pedal feel and straight-line stopping
• After road test, re-inspect all master cylinder connections for leaks and verify brake fluid level is at MAX line
• Check brake pedal feel and travel one final time - pedal should be firm with approximately 1-2 inches of free play before braking begins
• If ANY abnormality is detected (soft pedal, pulling, noise, warning lights), do NOT drive vehicle - seek professional diagnosis immediately

Related trouble codes on this vehicle

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