Voodoo Brake Science - Master Cylinder Pressure

131 9803 Voodoo Brake Science Proportioning Valve And Metering Valve

Since most of the braking is done by the front wheels, a proportioning valve is used to limit pressure to the rear brakes so they don't lock up prematurely. This factory unit also includes a metering valve for the front discs and a warning light switch in case pressure in the front or rear is lost, so it is called a combination valve. An adjustable proportioning valve can be a handy item for dialing in the amount of rear pressure on any custom brake setup, but it isn't always necessary.

Since most of the braking is done by the front wheels, a proportioning valve is used to li

Master Cylinder Pressure
Most hydraulic brake systems operate at a safe maximum of 1,000 to 1,200 psi, with regular braking pressures below this figure. The hydraulic pressure in the system is a function of input pressure from the pedal and the piston area of the master cylinder. The way to calculate this pressure (P) is to divide the force on the pedal (F) by the area of the piston face in square inches (A), or P=F÷A. Knowing that a 1-inch bore master cylinder has an area of 0.7854 square inches (šR2), and with a known force of 600 pounds on the plunger in the master cylinder, we come up with: 600÷0.7854=764 psi in the braking system. Likewise, substituting a larger or smaller diameter master cylinder bore in this scenario, such as 1 1/4-inch or 7/8-inch, will produce 489 psi or 997 psi, respectively. So a larger volume master cylinder (a bigger bore) will produce lower pressure than a master cylinder with a smaller bore.

Proper pressure, which is critical to the correct function of any braking system, can be adjusted as described with different size master cylinder bores and pedal ratios. A smaller master cylinder can be used if higher system pressure is needed. However, switching to a smaller master cylinder will also increase the pedal travel. If the system can't accommodate this travel (i.e., the pedal hits the floor), then this is not a good option. In that case it might be better to increase the force on the master cylinder by raising the pedal ratio to obtain more pressure in the system.

Master Cylinder Volume
When changing the diameter or the stroke of a master cylinder the volume of fluid displaced is changed as well. But since the volume of a closed system remains the same and brake fluid can't be compressed, the stroke of the cylinder is limited. On any brake system in proper operating condition the pads or the linings are very close to the rotors and drums, so not much movement is needed for them to make contact. After clearances are taken up, pressure rises in the system as the pedal is pushed down, but the actual fluid displacement within the system remains the same, regardless of the diameter of the master cylinder. The volume of fluid displaced equals the stroke of the piston times the surface area, so a larger-diameter master cylinder needs a smaller stroke to displace the same amount of fluid as a smaller-diameter one. This is one case where bigger isn't always better. If the stroke and the diameter of the master cylinder and the pedal ratio can't be changed, then adjusting the pressure within the system can sometimes be accomplished by using larger wheel cylinders or bigger calipers.

131 9803 Voodoo Brake Science Brake Fluid Bottle

Any brake system must contain the proper brake fluid and be completely bled of air. Even though the new silicon brake fluids don't absorb water like standard fluids, they do have a tendency to compress slightly and give a spongy feeling at extremely high temperatures, such as are encountered in racing. However, if you are plagued by corrosion and water contamination, silicon-based fluid is a good alternative.

Any brake system must contain the proper brake fluid and be completely bled of air. Even t

Here's a simple formula to determine the volume of displacement necessary to safely operate a brake system. Proper capacity is determined by the equation D=S x A, where D is the total cylinder displacement, S is the stroke, and A is the piston area in square inches. To see how this works, we'll use a theoretical four-wheel disc system where the front calipers each displace 0.075 cubic inch and the rears each displace 0.050 cubic inch. The total volume of all four calipers is 0.25 cubic inch. ([0.075 x 2]+[0.05 x 2]). After multiplying by a safety factor of 100 percent to account for swelling, leakage, and deflection, we arrive at a total of 0.5 cubic inches of needed cylinder displacement. The master cylinder stroke is 1.2 inches, a common length. Solving for the unknown variable, where D÷S=A, we get 0.5÷1.2=0.417, or a piston area of 0.417 square inch. That translates to a bore size of between 11/16 and 3/4 inch (Piston area=šR2).

The End Result
To design and build the perfect braking system for any particular truck, many additional factors must also be considered. The coefficient of friction of the pads or shoes, the brake torque of such, the vehicle's center of gravity and total weight, and so on. Do real people actually sit down and measure all these factors and figure out exactly how to build a brake system? You bet they do! But the old seat-of-the-pants method usually comes into play once the general design work is finished. Most builders set up a rig and then go out and test it on a deserted road with no nearby telephone poles or cliffs. Changing any or all of the aforementioned variables a little at a time is essential to dial-in proper braking for each individual vehicle, especially custom-modified ones.

If you don't know the diameter of your wheel cylinders, the answer is on the inside. Rather than measure the actual cylinder bore, look on the back of the cup; the diameter is usually molded into the surface of the rubber. These diameters are important should you want to calculate piston displacement or line pressures when troubleshooting or modifying a brake system. Many times, larger- or smaller-than-standard wheel cylinders can be swapped to correctly bias a system for better braking performance.
If you don't know the diameter of your wheel cylinders, the answer is on the inside. Rathe
Early 4x4s usually came with small and inefficient brakes, as illustrated by this 9-inch-diameter drum on an early Jeep. Later 11-inch drums provide greater surface area and stopping power, especially when combined with different-size wheel cylinders. If you've ever had to hold the brake pedal to the floor on the side of a hill to keep from rolling over, you know how important low pedal pressure and quality brakes can be.
Early 4x4s usually came with small and inefficient brakes, as illustrated by this 9-inch-d
Swapping master cylinders and boosters can be a chore if not done correctly. Almost any quality auto parts store can give you the specifications on master cylinders, including bore size, stroke, and mounting pattern. When swapping cylinders with different bore sizes, remember that pedal effort and travel will change. The same warning also applies to changing to a different diameter or style of power booster.
Swapping master cylinders and boosters can be a chore if not done correctly. Almost any qu

Caliper swapping can be used to bias a brake system or adjust it for a mismatched master cylinder, but a well-thought-out system should be planned before jumping in to fix a mistake. Rear disc systems can be especially troublesome if piston diameters are either too small or too large compared with the surface area of the original wheel cylinder.
Caliper swapping can be used to bias a brake system or adjust it for a mismatched master c
Most standard GM calipers for 4x4s are the same, but some two-wheel-drive GM calipers will fit the brackets and function reasonably well, even though they have different piston sizes. This can lead to unequal braking force, especially if they're on opposite sides of the same axle. However, proper bias of a system can be achieved by using different-size calipers in pairs on the front or rear axles, depending on what the system needs.
Most standard GM calipers for 4x4s are the same, but some two-wheel-drive GM calipers will
Aftermarket kits for disc brakes are usually properly matched to a specific vehicle application using the stock master cylinder. On some kits, a replacement master cylinder is available, as are adjustable proportioning valves to set the front-to-rear brake bias correctly. Proper biasing is when the rear brakes lock up just after the front ones during a panic stop.
Aftermarket kits for disc brakes are usually properly matched to a specific vehicle applic