Brake Proportioning Valve

Replace the brake proportioning valve to restore proper brake force distribution between front and rear wheels, critical for safe braking performance.

Warnings

Tools required

Parts

• Brake proportioning valve × 1 — Use OEM specification for 1970 AMC Rebel with disc/drum combination
• Brake line washers/seals (if applicable) × 1 — Check valve assembly for required seals
• DOT 3 or DOT 4 brake fluid × 1 — Use DOT 3 minimum

Fluids

• DOT 3 or DOT 4 brake fluid — 1 qt

Preparation

1. Park vehicle on level ground and chock rear wheels securely
2. Raise and support vehicle on jack stands if valve is located underneath
3. Locate the proportioning valve - typically mounted on driver side frame rail or firewall with multiple brake lines connected
4. Clean area around valve and brake line connections with wire brush to prevent contamination
5. Apply penetrating oil to brake line fittings and allow 10-15 minutes to soak if corrosion present
6. Pump brake pedal several times to relieve residual pressure
7. Use turkey baster or fluid pump to remove brake fluid from master cylinder reservoir to minimize spillage
8. Prepare brake line caps or rubber plugs and have them immediately accessible
9. Place drain pan under work area

Procedure

1. 1

   Label and disconnect inlet brake line
   Identify the inlet line from the master cylinder (typically single line entering valve). Mark this line with tape for identification. Using flare nut wrench to prevent rounding, loosen and remove inlet brake line fitting from proportioning valve. Immediately cap or plug both the line and valve port to prevent fluid loss and contamination. Allow fluid to drain into pan.
   ⚠Use flare nut wrench only - standard wrenches will round soft brake line fittings and create dangerous leaks.
2. 2

   Disconnect rear brake lines
   The Rebel Machine proportioning valve typically has two outlet ports for rear brakes. Label each line for left and right identification. Using flare nut wrench, carefully loosen and remove each rear brake line from valve. Cap or plug each line and valve port immediately. Support brake lines to prevent bending or kinking.
3. 3

   Remove proportioning valve mounting hardware
   Locate valve mounting bracket bolts or clips. The valve is typically held by 1-2 bolts to frame rail or firewall bracket. Remove mounting hardware while supporting valve. Note orientation and position of valve and any mounting brackets for correct reinstallation.
4. 4

   Remove old proportioning valve
   Carefully remove old valve from mounting location. Inspect mounting area for corrosion or damage. Clean mounting surface thoroughly. Inspect old valve for signs of internal failure such as external leaking, corrosion, or damage to ports.
5. 5

   Prepare new proportioning valve
   Inspect new valve to confirm correct application for 1970 Rebel Machine disc/drum brake system. Verify port sizes match original. Check that any seals or washers are properly installed in ports. Do not remove protective caps until ready to connect lines.
6. 6

   Install new proportioning valve
   Position new valve in same orientation as original with ports aligned for brake line connection without strain. Install mounting hardware and tighten securely to frame or firewall bracket. Ensure valve is solidly mounted with no movement. Verify adequate clearance for brake lines and that valve is positioned to prevent line kinking.
7. 7

   Connect inlet brake line
   Remove cap from master cylinder line and valve inlet port. Inspect line flare for damage. Thread inlet line fitting into valve by hand first to ensure proper thread engagement and prevent cross-threading. Once hand-tight, use flare nut wrench to snug fitting. Tighten to 10-12 ft-lbs (132-156 in-lbs) - firm but not excessive force to avoid damaging flare.
8. 8

   Connect rear brake lines
   Remove caps from rear brake lines and corresponding valve outlet ports. Match each line to correct port using labels applied during removal. Thread each fitting by hand first to start threads properly. Once hand-tight, use flare nut wrench to snug each fitting to 10-12 ft-lbs. Double-check that lines are routed without kinks or strain on fittings.
9. 9

   Refill master cylinder
   Remove master cylinder reservoir cap. Fill reservoir with fresh DOT 3 or DOT 4 brake fluid to MAX line. Keep reservoir covered when not actively filling to prevent moisture contamination. Have additional fluid ready for bleeding process.
   ⚠Use only fresh brake fluid from sealed container. Never reuse drained fluid.
10. 10

    Bleed brake system completely
    Bleed entire brake system starting with wheel furthest from master cylinder (right rear), then left rear, right front, left front. At each wheel: attach clear hose to bleeder screw into container, open bleeder 1/2 turn, have assistant slowly press brake pedal to floor, close bleeder before pedal rises, repeat until fluid runs clear without bubbles. Check master cylinder level frequently and maintain above MIN line throughout process. Pump brake pedal between wheels to restore pressure.
    ⚠️Air in brake system will cause brake failure. Continue bleeding until absolutely no air bubbles appear in fluid from any wheel.
11. 11

    Inspect all connections for leaks
    With reservoir filled to MAX, have assistant apply firm steady pressure to brake pedal and hold for 30 seconds. Carefully inspect each brake line connection at proportioning valve for any sign of fluid seepage. Check mounting area under valve for drips. If any leak detected, immediately release pedal pressure, repair leak, and re-bleed system completely.
    ⚠️Any brake fluid leak, no matter how small, is a critical safety hazard and must be repaired before operating vehicle.

Reassembly

1. Verify master cylinder reservoir is filled to MAX line with fresh brake fluid
2. Confirm all brake line connections are tight and leak-free
3. Ensure proportioning valve is securely mounted with no movement
4. Clean any spilled brake fluid from frame, components, and painted surfaces immediately
5. If vehicle was raised, lower from jack stands and remove chocks

Verification

• With engine off, pump brake pedal multiple times - pedal should build firm resistance and hold steady pressure without sinking
• Start engine (power brakes) - pedal should drop slightly then hold firm with no further sinking
• Have assistant apply firm brake pressure while you inspect all proportioning valve connections for leaks
• Perform low-speed test in safe area - vehicle should brake smoothly without pulling to either side or rear wheel lockup
• Perform several moderate brake applications from 25-30 mph in safe area - rear brakes should engage proportionally without premature lockup
• Check brake pedal feel - should be firm within upper 1/3 of pedal travel with no sponginess indicating air in system
• Any abnormal behavior, soft pedal, or leaks requires immediate professional inspection before road use

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