Brake Booster

Remove and replace the vacuum-operated brake booster on a 1970 AMC Rebel Machine to restore proper brake assist function.

Warnings

Tools required

Parts

• Brake booster assembly × 1 — Use OEM specification for 1970 AMC Rebel with 390 V8
• Brake booster mounting nuts × 4 — 3/8" mounting hardware
• Vacuum hose (if deteriorated) × 1 — 3/8" vacuum hose
• Master cylinder gasket × 1 — Use OEM specification
• Pushrod retaining clip × 1 — Use OEM specification

Fluids

• DOT 3 or DOT 4 brake fluid — 1 qt

Preparation

1. Park vehicle on level ground and set parking brake firmly
2. Disconnect negative battery cable to prevent accidental electrical shorts
3. Allow engine to cool completely if recently operated
4. Open hood and locate brake booster on firewall behind master cylinder
5. Place absorbent rags or catch pan under master cylinder to catch brake fluid spillage
6. Spray penetrating oil on all brake booster mounting nuts and vacuum hose connection, allow to soak 15 minutes
7. Have a helper available for accessing components from inside the vehicle

Procedure

1. 1

   Remove air cleaner assembly
   Remove the air cleaner assembly from the 390 V8 to provide clearance for accessing the brake booster. Remove wing nut, lift off air cleaner lid, then remove the entire air cleaner base by disconnecting any vacuum hoses and removing mounting hardware. Set aside in a clean area.
2. 2

   Disconnect vacuum hose from booster
   Locate the vacuum hose connecting the intake manifold to the brake booster check valve on the front of the booster. The check valve is a one-way valve that prevents loss of vacuum. Carefully twist and pull the vacuum hose from the check valve. If the hose is brittle or cracked, plan to replace it. Remove the check valve from the booster by twisting counterclockwise.
3. 3

   Disconnect brake lines from master cylinder
   Using a flare nut wrench to prevent rounding, carefully loosen the two brake line fittings at the master cylinder. The front line feeds the front brakes, the rear line feeds the rear brakes. Once loose, unthread the fittings by hand and immediately plug the brake lines with appropriate caps or plugs to prevent fluid loss and contamination. Have rags ready to catch dripping brake fluid.
   ⚠Use only a flare nut wrench on brake line fittings. Standard wrenches will round off the soft brass fittings.
4. 4

   Remove master cylinder from booster
   Locate the two nuts securing the master cylinder to the brake booster studs. These are typically 1/2" nuts. Remove both nuts and carefully pull the master cylinder straight off the booster studs. The master cylinder pushrod will remain in the booster. Set the master cylinder aside in an upright position to minimize fluid loss. Note the orientation of any shims or spacers between the master cylinder and booster.
5. 5

   Access booster pushrod from inside vehicle
   Move to the driver's side interior. Remove the lower dash panel if necessary to access the area above the brake pedal. Locate where the brake booster pushrod connects to the brake pedal arm. You will see a clevis pin with a retaining clip securing the pushrod to the pedal.
6. 6

   Disconnect pushrod from brake pedal
   Using pliers or a small screwdriver, remove the retaining clip from the clevis pin that secures the booster pushrod to the brake pedal arm. Note the orientation of the clip for reassembly. Once the clip is removed, slide out the clevis pin. The pushrod is now disconnected from the pedal. You may need to use a flashlight to see clearly in this confined area.
   ⚠The retaining clip can spring away when removed. Work carefully to avoid losing small parts in the vehicle interior.
7. 7

   Remove booster mounting nuts
   From inside the vehicle, locate the four mounting nuts on the studs that protrude through the firewall. These are typically 9/16" nuts. Using a socket or wrench, remove all four nuts. Keep these nuts as they will be reused unless damaged. Note that some fasteners may be difficult to access due to the steering column location.
8. 8

   Remove brake booster assembly
   Return to the engine compartment. With all mounting nuts removed and pushrod disconnected, carefully pull the brake booster assembly straight back away from the firewall. The unit is heavy (approximately 15 pounds) and the pushrod will come through the firewall with it. Maneuver the booster out of the engine compartment, being careful not to damage surrounding components or wiring. Inspect the firewall mounting surface for damage or rust.
9. 9

   Prepare new booster for installation
   Remove the new brake booster from packaging. Compare it to the old unit to verify correct application. Check that the pushrod length is appropriate - it should extend approximately 7.5-8.0 inches from the mounting surface. Install the check valve into the new booster by threading it clockwise until snug. Do not overtighten as the booster housing can crack.
10. 10

    Install new booster to firewall
    From the engine compartment, carefully guide the new booster into position, inserting the pushrod through the firewall opening first. Align the four mounting studs with the holes in the firewall. You may need a helper to guide the pushrod to the brake pedal connection point from inside the vehicle. Press the booster firmly against the firewall to ensure proper seating.
11. 11

    Secure booster mounting nuts
    From inside the vehicle, thread all four mounting nuts onto the studs by hand to start. Once all nuts are started, tighten them in a cross pattern to ensure even seating against the firewall. Tighten firmly but do not over-torque, as this can distort the booster housing. Approximately 15-20 ft-lbs is typical, though no specification is provided in the database.
12. 12

    Reconnect pushrod to brake pedal
    Align the pushrod clevis with the hole in the brake pedal arm. Insert the clevis pin through both the clevis and pedal arm. Secure with the retaining clip in its original orientation. Verify the pin cannot slide out by attempting to move it. The connection should be secure with no lateral play.
13. 13

    Install master cylinder
    Install a new master cylinder gasket onto the booster studs. Verify the master cylinder pushrod in the booster is properly seated. Carefully align the master cylinder with the booster studs and push it straight onto the studs until it contacts the gasket. Install the two mounting nuts and tighten in an alternating pattern. Tighten firmly to ensure a vacuum-tight seal between the booster and master cylinder.
14. 14

    Reconnect brake lines
    Remove the plugs from the brake lines. Thread the brake line fittings into the master cylinder ports by hand to avoid cross-threading. Once hand-tight, use a flare nut wrench to snug the fittings. Do not overtighten as this can damage the brass fittings or master cylinder ports. Typical torque is 10-15 ft-lbs but avoid excessive force.
15. 15

    Reconnect vacuum hose and bleed brakes
    Connect the vacuum hose to the check valve on the booster, ensuring a tight seal. Reconnect the battery. Fill the master cylinder reservoir with fresh DOT 3 or DOT 4 brake fluid. Bleed the entire brake system starting with the wheel furthest from the master cylinder (right rear, left rear, right front, left front). Refill the master cylinder frequently during bleeding to prevent introducing air. Continue bleeding until firm pedal is achieved.
    ⚠️The brake system must be completely bled of air before operating the vehicle. Air in brake lines will cause brake failure.

Reassembly

1. Reinstall air cleaner assembly and secure all vacuum hoses
2. Reinstall lower dash panel if removed during interior work
3. Top off master cylinder reservoir to proper level with fresh brake fluid
4. Verify all brake line connections are tight and showing no leaks
5. Check that brake pedal has firm resistance and does not sink to floor

Verification

• With engine off, pump brake pedal several times - it should become firm and hold pressure without sinking
• Start engine - brake pedal should drop slightly as booster vacuum assist activates, indicating proper booster function
• With engine running, press brake pedal firmly and hold for 30 seconds - pedal should not sink, indicating no leaks in system
• Test drive in safe area at low speed, verifying brakes engage smoothly and vehicle stops in straight line without pulling
• Check for vacuum leaks by listening for hissing sounds at booster connections with engine running
• Inspect all brake line connections for fluid seepage after test drive
• Verify master cylinder reservoir level has not dropped, indicating no leaks in system
• Confirm brake warning light is not illuminated on instrument panel

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