It's Proportional

Have you ever slammed on your hot rod's brakes really hard? Did the rear brakes lock up before the front ones? If so, then you're familiar with that adrenaline-pumping feeling when the tail end of your car tries to become the front end at speed. It’s not fun and certainly not safe. In our recent 0-100-0 shootout (Slam Dance), we witnessed several cars experience this slide-for-life dilemma, so we thought we’d research the proper way to prevent this with the installation of a rear brake proportioning valve.

If you’ve been reading this magazine for a while, then you know what’s coming next. Before we dive into the how-to, we need to review a few brake basics. Hydraulic brakes work on a very simple principal: Load is applied to the brake pedal, which creates pressure using hydraulic fluid that is directed through high-pressure lines to either a drum wheel cylinder or a disc brake caliper. For disc brakes, this pressure (in pounds per square inch—psi) is applied to one or more pistons that squeeze brake pads against a rotating disc. For drum brakes, the pistons in the wheel cylinders push outward to apply shoes against a rotating drum. Drum brakes benefit from something called self-energizing action where the leading shoe uses the rotating drum to help push the larger trailing shoe into the drum with additional force, requiring less hydraulic pressure to apply the brakes.

After disc brakes began appearing on production cars, the factory came up with a device called a combination valve. This valve is placed downstream of the master cylinder and performed both as a proportioning valve to reduce the pressure to the rear brakes while also acting as a failure warning device to trigger a light should a loss in hydraulic pressure occur. Factory combination valves are carefully engineered to reduce pressure to the rear brakes to balance the braking effort for maximum braking efficiency. Unfortunately, this device is not adjustable, which means any change to brake or suspension components may create a premature rear-brake lockup problem. Ideally, the front and rear brakes lock up at the same time.

Whether the car is equipped with front disc and rear drums, or has four-wheel discs, hydraulic pressure should be reduced to the rear brakes for several reasons. The single most important reason is because even under moderate braking, natural weight transfer reduces weight on the rear tires and moves the weight to the front. This unloads the rear tires, which makes it much easier to lock up the rear brakes in hard braking. Once the rear brakes are locked, the tires skid, which immediately causes loss of control.

Since we deal with modified cars, even something as simple as adding taller rear tires can have a drastic effect on braking performance. Taller tires increase the distance between the rear axle centerline and the ground, increasing the leverage of the brakes over the tires. This was most evident with Tim Moore’s car on the 0-100-0 shootout last month when, even though he carefully set the brake balance for his shorter set of tires, the taller drag slicks caused a rear brake lockup problem. Moore solved the problem by reducing the rear brake pressure by several hundred psi.

Most adjustable brake proportioning valves are very simple devices designed to fit between the master cylinder and the rear brake line. The valve offers a range of adjustment that decreases master-cylinder pressure down to a preset minimum. Most valves reduce the master-cylinder pressure by turning the valve clockwise.

The most difficult part of installing a brake proportioning valve is making a new line or two to place the proportioning valve in the hydraulic system. This usually only requires cutting the rear line and making a couple of 45-degree flares on each end of the tubing to create a leak-proof seal. The best place to put the proportioning valve is adjacent to the master cylinder where it is easily accessible.

Once the valve is in place, you should first test to ensure that all air is bled out of the hydraulic system. A soft or spongy brake pedal is a good indicator of air in the system that must be bled in order to create maximum brake performance. Once the system is properly bled, it’s time to set the proper brake-pressure bias. The safest procedure is to take the car to a long, wide stretch of highway with no traffic where you can safely perform repeated 60-0–mph brake tests.

Before testing, be sure to minimize the rear brake pressure by turning the proportioning valve completely counterclockwise. At slow speeds (around 30 mph) perform several tests to ensure that the brakes work properly. Since the front brakes are responsible for the majority of brake performance, the car should still stop safely at slow speeds. Crank the prop valve two turns and perform the 30-mph test again. Keep increasing rear brake pressure until the rears lock up before the front brakes, then return to the highest rear brake pressure setting that did not lock up the rear brakes first.

Once these low-speed tests have been finalized, test the brakes at 50 mph, >> applying brake pressure gradually at first and then more aggressively to determine that the rear brakes do not lock up first. Remember to give the brakes a chance to cool by driving at a steady speed for a mile or so after each test. If the rear brakes begin to lock up, immediately reduce brake pressure to prevent loss of control. Just because the rear brakes don’t lock up first at 30 mph doesn’t eliminate the chance that they might lock up at 60 mph in an emergency-stop situation. It is critical that the rear brakes do not lock up first under any circumstances since this could lead to a loss of control. Obviously, with enough brake pressure, all four wheels can lock up, but we are looking for the rears to lock up only after the front brakes do.

You don’t have to be a vehicle-dynamics expert to recognize that brakes that perform at their maximum potential are an important factor in any car. And since hot rodders like to go fast, this means stopping is even more important. A properly installed and adjusted brake proportioning valve can dramatically improve stopping distance, not to mention bolster your confidence. When it comes to stopping safely, nobody wants to be slip sliding away.