Brake Master Cylinder

Remove and replace the brake master cylinder on a 1970 AMC Rebel Machine, including brake line disconnection, mounting hardware removal, and system bleeding.

Warnings

Tools required

Parts

• Brake master cylinder × 1 — Use OEM specification or quality aftermarket equivalent for dual reservoir master cylinder
• Master cylinder mounting gasket or seal × 1 — Check if included with new master cylinder
• Brake line copper washers or seals × 1 — Various sizes for all brake line connections

Fluids

• DOT 3 or DOT 4 brake fluid — 2 qt

Preparation

1. Park vehicle on level surface and engage parking brake. Chock rear wheels securely.
2. Verify you have sufficient fresh brake fluid from sealed containers before beginning work.
3. Clean the master cylinder, brake lines, and surrounding firewall area with brake cleaner to prevent dirt contamination during disassembly.
4. Identify all brake line connections on the master cylinder. The 1970 Rebel Machine uses a dual reservoir master cylinder with typically two or four brake line connections.
5. Place drain pan or absorbent material under the master cylinder area to catch brake fluid spillage.
6. Use turkey baster or fluid syringe to remove as much brake fluid as possible from the master cylinder reservoir to minimize spillage.

Procedure

1. 1

   Disconnect brake lines from master cylinder
   Using the appropriate size flare nut wrench (typically 3/8" or 7/16"), carefully loosen each brake line fitting at the master cylinder. Work slowly to avoid rounding off the soft brass fittings. Once loose, unscrew by hand and immediately plug or cap the brake line ends to prevent fluid loss and contamination. Mark or photograph line positions for correct reinstallation.
2. 2

   Disconnect pushrod from brake pedal
   Working inside the vehicle under the dashboard, locate the master cylinder pushrod connection to the brake pedal arm. Remove the retaining clip or cotter pin securing the clevis pin, then slide the clevis pin out to disconnect the pushrod from the pedal. Note the pushrod adjustment and orientation for reassembly.
3. 3

   Remove master cylinder mounting nuts
   From the engine compartment, locate the two mounting nuts securing the master cylinder to the firewall or brake booster (if equipped). These are typically 1/2" or 9/16" nuts. Remove both mounting nuts completely. Support the master cylinder to prevent it from falling.
4. 4

   Remove old master cylinder
   Carefully pull the master cylinder forward off the mounting studs and away from the firewall. Tilt as needed to clear any obstructions. Have rags ready to catch any remaining brake fluid. Inspect the firewall mounting area and pushrod boot for damage or deterioration.
5. 5

   Prepare new master cylinder
   Bench bleed the new master cylinder before installation to remove internal air. Secure the cylinder in a vise (protect finish with soft jaws), fill both reservoirs with fresh brake fluid, install temporary bleeder tubes from outlet ports back into reservoir, and slowly cycle the pushrod until no air bubbles emerge from either port. This critical step prevents air from entering the vehicle brake system.
6. 6

   Install new mounting gasket or seal
   Place the new master cylinder mounting gasket or seal onto the mounting studs at the firewall. Ensure it is properly positioned and not torn or damaged. Some applications use a rubber boot on the pushrod instead of a separate gasket.
7. 7

   Install new master cylinder
   Carefully guide the new master cylinder onto the mounting studs, ensuring the pushrod enters the master cylinder bore properly and the gasket remains in position. Push the cylinder fully against the firewall or brake booster mounting surface. Thread on both mounting nuts and tighten evenly in a cross pattern to secure the cylinder firmly without over-torquing (typically 18-25 ft-lbs, though no specification is available for this vehicle).
8. 8

   Reconnect pushrod to brake pedal
   From inside the vehicle, reconnect the master cylinder pushrod to the brake pedal clevis. Install the clevis pin and secure with the retaining clip or new cotter pin. Verify the connection is secure and the pushrod moves freely without binding.
9. 9

   Connect brake lines to master cylinder
   Remove the temporary plugs from the brake lines. Install new copper washers or seals if applicable. Thread each brake line fitting into the correct master cylinder port by hand first to prevent cross-threading. Once hand-tight, use the flare nut wrench to snug the fittings firmly (typically 10-15 ft-lbs for brake line fittings). Do not over-tighten as this can damage the soft brass fittings and cause leaks.
10. 10

    Fill master cylinder reservoir
    Fill both chambers of the master cylinder reservoir with fresh DOT 3 or DOT 4 brake fluid to the proper fill level. Use only new fluid from a sealed container. Keep the reservoir filled throughout the bleeding process.
11. 11

    Bleed brake system at all four wheels
    Bleed the brake system starting with the wheel furthest from the master cylinder (typically right rear, then left rear, then right front, then left front). At each wheel, attach clear tubing to the bleeder screw with the other end submerged in clean brake fluid in a container. Have an assistant pump the brake pedal several times and hold pressure. Open the bleeder screw, allow fluid and air to escape, then close before the pedal reaches the floor. Repeat until no air bubbles emerge. Keep the master cylinder reservoir filled at all times during this process.
12. 12

    Check for leaks and proper operation
    With the system bled, inspect all brake line connections at the master cylinder for any signs of fluid leakage. Check the mounting area for leaks around the pushrod seal. Start the engine if equipped with power brakes and verify the pedal drops slightly. The brake pedal should feel firm with no sponginess when depressed.

Reassembly

1. Double-check that all brake line fittings are properly tightened and not leaking.
2. Ensure the master cylinder reservoir is filled to the proper level with fresh brake fluid and the cap is securely installed.
3. Verify the pushrod connection at the brake pedal is secure with the retaining clip or cotter pin properly installed.
4. Clean any spilled brake fluid from painted surfaces immediately to prevent damage.

Verification

• With the engine running (if power brakes) or off (if manual), pump the brake pedal several times. It should become firm and hold steady pressure without sinking to the floor.
• Inspect all connections at the master cylinder for any signs of brake fluid leakage while pressure is applied to the pedal.
• Test the brakes at low speed in a safe area before returning the vehicle to normal service. The vehicle should stop smoothly and in a straight line without pulling.
• After the initial test drive, recheck the master cylinder reservoir fluid level and top off if needed. Recheck all fittings for leaks.
• Road test at progressively higher speeds only after confirming proper brake operation at low speeds. If the pedal feels soft, spongy, or requires excessive travel, re-bleed the system.

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