Brake Proportioning Valve

Replace or install the brake proportioning valve on a 1966 Corvette to restore proper front-to-rear brake bias and prevent premature rear wheel lockup under braking.

Warnings

Tools required

Parts

• Brake proportioning valve × 1 — Use OEM specification or Corvette-specific aftermarket
• Brake line copper crush washers (if applicable) × 4 — Use OEM specification

Fluids

• DOT 4 Brake Fluid — 1 qt

Preparation

1. Park vehicle on level, solid ground and engage parking brake firmly.
2. Chock front wheels securely to prevent vehicle movement.
3. Raise rear of vehicle and support on jack stands rated for vehicle weight at approved jacking points on frame rails.
4. Locate the proportioning valve - typically mounted on left frame rail behind master cylinder or on rear axle housing near differential.
5. Identify all brake lines connected to the proportioning valve (typically 4 lines: front inlet from master cylinder, rear outlet to rear brakes, and two front outlets).
6. Clean area around proportioning valve and all brake line fittings with wire brush and shop rags to prevent contamination during disconnection.
7. Apply penetrating oil to all brake line fittings at proportioning valve and allow to soak for 10-15 minutes if corrosion is present.
8. Place drain pan under work area to catch brake fluid.
9. Have fresh DOT 4 brake fluid ready and ensure master cylinder reservoir cap is accessible for refilling during bleeding.

Procedure

1. 1

   Relieve residual brake pressure
   With engine off, pump brake pedal 15-20 times to relieve any residual vacuum assist pressure in the system. This reduces initial fluid loss when lines are disconnected.
2. 2

   Label and disconnect inlet line from master cylinder
   Using the appropriate flare nut wrench (typically 7/16"), carefully loosen the brake line fitting connecting the master cylinder outlet to the proportioning valve inlet. This is usually the line coming from the front of the vehicle. Mark this line with tape labeled 'INLET' to ensure correct reinstallation. Allow fluid to drain into pan. Once loose, unthread completely by hand to avoid cross-threading.
3. 3

   Disconnect front brake circuit lines
   Identify the two outlet lines going to the front brake circuits (left and right front). Using flare nut wrenches, loosen and remove both front circuit line fittings from the proportioning valve. Label these lines 'FRONT LEFT' and 'FRONT RIGHT' accordingly. Cap or plug open line ends with rubber caps or clean rags to minimize fluid loss and contamination.
4. 4

   Disconnect rear brake circuit line
   Locate and disconnect the outlet line going to the rear brake circuit. This line typically runs along the frame rail toward the rear axle. Use the appropriate flare nut wrench to loosen the fitting at the proportioning valve. Label this line 'REAR' and cap the open end.
5. 5

   Remove proportioning valve mounting hardware
   Remove the mounting bracket bolt(s) securing the proportioning valve to the frame rail or axle housing. The valve is typically held by 1-2 bolts through a mounting bracket. Remove the old proportioning valve from the vehicle and inspect the mounting bracket for damage or corrosion.
6. 6

   Prepare new proportioning valve
   Remove the new proportioning valve from packaging and verify all ports are clean and free of debris. If the valve has removable port plugs, ensure they are present and properly seated. Check that any included copper crush washers or sealing washers are in place at each port. Compare the new valve to the old valve to confirm correct configuration and port arrangement.
7. 7

   Install new proportioning valve
   Position the new proportioning valve in the mounting bracket ensuring proper orientation (inlet port toward master cylinder, outlets toward brake circuits). Install mounting bracket bolt(s) and tighten firmly but do not overtorque - typical spec is 120-180 inch-lbs but verify for your specific valve. Ensure valve is solidly mounted with no movement.
8. 8

   Reconnect inlet line from master cylinder
   Thread the inlet line from the master cylinder into the proportioning valve inlet port by hand first to ensure proper thread engagement and avoid cross-threading. Once hand-tight, use flare nut wrench to snug the fitting. Typical torque for 3/16" brake line fittings is 120-180 inch-lbs - tighten firmly but do not overtorque as this can damage the flare and cause leaks.
9. 9

   Reconnect front brake circuit lines
   Connect the front left and front right brake lines to their respective ports on the proportioning valve, referencing your labels from disassembly. Thread fittings by hand first, then tighten with flare nut wrench to 120-180 inch-lbs. Ensure each line is routed without kinks or contact with moving parts or exhaust.
10. 10

    Reconnect rear brake circuit line
    Connect the rear brake circuit line to the rear outlet port on the proportioning valve. Thread by hand first, then tighten with flare nut wrench to 120-180 inch-lbs. Verify the line has adequate slack and will not be stressed during suspension travel.
11. 11

    Bleed brake system at proportioning valve
    Fill master cylinder reservoir to MAX line with fresh DOT 4 brake fluid. If the proportioning valve has a bleeder screw, open it slightly and have an assistant slowly pump the brake pedal until fluid flows freely without air bubbles, then close bleeder while pedal is held down. If no bleeder is present, proceed to bleeding at each wheel. Monitor master cylinder level constantly - never let it run dry.
12. 12

    Bleed all four wheels in correct sequence
    Bleed brakes in the following sequence: right rear, left rear, right front, left front (furthest to closest from master cylinder). At each wheel, attach clear bleeder hose to bleeder screw and submerge other end in clean brake fluid in a container. Have assistant pump pedal 3-4 times and hold down, then open bleeder screw until fluid flows, close bleeder while pedal is still down, then release pedal. Repeat until no air bubbles appear in fluid stream. Check and refill master cylinder after each wheel.
13. 13

    Perform final system check
    With vehicle still on jack stands and engine off, pump brake pedal firmly 20-30 times. Pedal should become firm and hold pressure without sinking. If pedal remains soft or sinks, air remains in system and re-bleeding is required. Inspect all connections at proportioning valve for any signs of fluid seepage or leaks. Tighten any weeping fittings an additional 1/8 turn.

Reassembly

1. Top off master cylinder reservoir to MAX line with fresh DOT 4 brake fluid and install reservoir cap securely.
2. Remove jack stands and carefully lower vehicle to ground.
3. Remove wheel chocks.
4. Wipe down all brake components and frame area to remove any spilled brake fluid residue.
5. Dispose of old brake fluid properly at an approved collection facility - do not pour down drain or on ground.

Verification

• With engine running (to activate power brakes), pump pedal several times and verify firm, consistent pedal feel with no sinking or sponginess.
• Perform a low-speed brake test in a safe area (empty parking lot): from 10-15 mph, apply brakes firmly and verify vehicle stops in a straight line with no pulling to either side and no premature rear wheel lockup.
• Perform a moderate-speed brake test in a safe area: from 25-30 mph, apply brakes progressively and verify smooth, controlled stops with proper front-to-rear brake balance (front brakes should engage slightly before rears).
• Re-inspect all proportioning valve connections after test drive for any signs of fluid leakage. Address any leaks immediately before returning vehicle to service.
• Check brake fluid level in master cylinder reservoir - should be at MAX line. Top off if needed.
• Re-check brake pedal feel after test drive. If pedal becomes soft or spongy after initial firm feel, air remains in system and re-bleeding is required.
• CRITICAL: If brake pedal is soft, spongy, or sinks under steady pressure, or if vehicle pulls severely to one side, DO NOT drive vehicle. System requires further bleeding or repair. Have vehicle towed to qualified brake technician.

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