If you have recently converted your rear drum brakes to discs, or if your track car's rear end tries to "pass" you every time you slam on the brakes, you are experiencing incorrect brake bias. Your factory combination valve was designed for a specific weight distribution and brake setup. Once you change tires, lower the suspension, or upgrade calipers, that factory calibration is useless.
This is where the Adjustable Proportioning Valve comes in. It is the only way to fine-tune your brake balance to prevent rear wheel lockup and minimize stopping distances.
How It Works: The "Knee Point" Concept
Many people think turning the knob changes the percentage of braking force (e.g., from 60/40 to 70/30). This is incorrect. An adjustable proportioning valve acts as a pressure regulator with a set point, known as the "Knee Point."
- Before the Knee Point: The valve is wide open. Input pressure = Output pressure (1:1 ratio). Rear brakes get full power.
- After the Knee Point: The valve restricts flow. For every 100 psi of extra pedal pressure, the rear brakes might only see 43 psi increase (a 57% reduction).
Turning the knob moves this Knee Point:
- Knob Out (Less Bias): The restriction starts early (e.g., at 300 psi). Rear brakes are weak.
- Knob In (More Bias): The restriction starts late (e.g., at 1000 psi). Rear brakes do more work.
Most aftermarket valves (Wilwood, Summit, Jegs) have a fixed slope reduction of roughly 57%. You are adjusting when the reduction begins (the pressure threshold), not how much it reduces the slope.
Selection Guide: Knob vs. Lever Style
When shopping for a valve, you will encounter two main styles. Use this comparison to select the correct unit for your application.
| Feature | Knob Style (Screw Type) | Lever Style |
|---|---|---|
| Best For | Street cars, Hot Rods, Daily Drivers | Dedicated Race Cars, Rally, Drifting |
| Adjustment | Infinite adjustment (Fine tuning) | 6-7 distinct "click" positions (Repeatable) |
| Pros | Very compact, inexpensive | Adjustable blindly mid-race; repeatable settings |
| Cons | Hard to recall exact setting; hard to adjust while driving | Bulkier; expensive; overkill for street |
| Cost Est. | $40 - $80 | $100+ |
Installation & Plumbing Diagram
Never install a proportioning valve on the front brake line. It must ONLY go on the rear line.
- Master Cylinder
- Distribution Block (if equipped)
- Adjustable Proportioning Valve (IN port)
- Adjustable Proportioning Valve (OUT port)
- Rear Flex Hose/Axle T-fitting
Note on Factory Valves: If your car has a factory "Combination Valve," you must remove the factory proportioning valve internals or bypass it entirely. You cannot put an adjustable valve after a factory valve—the pressure will already be too low to adjust.
How to Tune Your Valve (Step-by-Step)
Once installed, you cannot just "set it and forget it." You must tune it to your specific car. Find a safe, dry, open parking lot or track. The goal is for the front wheels to lock up just before the rear wheels.
- Start "Full Decrease": Turn the knob all the way Counter-Clockwise (Screw OUT). This provides the least amount of rear brake pressure. This is your safe baseline.
- Test Run 1: Drive at 30 mph and brake hard. The front wheels should lock up (skid), but the rears should keep rolling. The car will stop straight.
- Adjust: Turn the knob Clockwise (Screw IN) 2 full turns. This increases rear pressure.
- Repeat: Do another run. If rears don't lock, add more turns.
- Find the Limit: Eventually, the rear wheels will lock up first. The car tail will fishtail or slide sideways. This is dangerous.
- Back Off: Once you find that lockup point, turn the knob Counter-Clockwise (Back out) 1 full turn.
Result: Under panic braking, your fronts will lock first (understeer), which is stable, while your rears provide maximum deceleration without losing traction.
Troubleshooting Common Issues
| Symptom | Probable Cause | Solution |
|---|---|---|
| Pedal feels spongy after install | Air trapped in valve body. | Turn knob all the way Clockwise (Full Increase) before bleeding. If restricted, air gets trapped behind the piston. |
| Rear brakes won't lock up at all | Insufficient system pressure or pad friction. | Check master cylinder size or rear pad compound. The valve can only reduce pressure, not create it. |
| Mounting inside cabin? | Legal restrictions. | Technically possible and common in racing, but check local laws (e.g., UK IVA) which may prohibit cockpit adjustment for street cars. |
