The Dragzine Network

Proper Hydraulic Pressure Is The Root To A Good Brake System

In some theories, a brake “system” is comparable to a racing engine. A racer concentrates on an accumulation of components to build a high-horsepower engine, but the wrong choice for a cam, timing, or many other elements can result in much less available power.

The same goes for the brakes to stop your hot rod. Components from here, there and anywhere may provide you with brakes, but is it a proper system that will adequately stop all that rolling inertia once it hits the dragstrip finish line?

Photos by Artie Maupin, Todd Silvey, and courtesy of manufacturers

The most prominent concept when working with a race car brake system is the measurement and monitoring of your hydraulic pressures inside your brake lines. Your brake lines are nothing magic; they transfer pressurized brake fluid from the master cylinder to your brake calipers. Short of a pinched line or leakage, their function is simplistic.

The determining factor for proper stopping power is what happens before and after the said brake line. The starting point of the brake pedal and master cylinder is one half of the combination; the calipers plus the related pads and rotors make up the critical end-game of your system.

Three physical laws govern the relationship between master cylinder and calipers; those rules include brake volume, brake pressure, and brake balance. Though our topic is brake pressure, these three points must work in orchestration, just like the racing engine mentioned above.

A typical drag racing brake system uses a four-piston caliper in the rear and a two-piston caliper on the front as shown by TBM Brakes. Other brake applications, such as the Generation 5 / COPO Camaro front brakes from Mark Williams utilize a four-piston front brake caliper thus requiring different master cylinder specifications.

We talked to two experts in race car braking, tapping the knowledge of Randy Cotteleer with The Brake Man (TBM) Brakes and Andrew Dickson from Mark Williams Enterprises (M-W). Each of these manufacturers specializes in hardcore brake components for drag racing.

“The key ingredient to any brake system is line pressure,” Dickson comments. “That pressure determines the clamping force of the calipers. We recommend at least 1,200 psi as the ultimate line pressure under severe braking conditions.”

Most brake manufacturers offer a chart as their starting point to set up your brake system based on the volume of fluid used to apply various sizes and quantities of brake pistons inside of the calipers. As an example, we will use a familiar illustration of a doorslammer specification that typically utilizes a four-piston caliper in the rear and a two-piston caliper on the front.

In this case of six pistons between our four brake corners, both TBM and Mark Williams recommend a 1-inch master cylinder to provide the proper brake fluid volume based on their individual brake caliper designs. Extremes such as just a pair of rear calipers on a dragster or using six-piston calipers all around in a road race car will dictate entirely different master cylinder specifications.

Most brake manufacturers offer a chart as their starting point to set up your brake system based on the volume of fluid used to apply various sizes and quantities of brake pistons inside of the calipers. As an example, we will use a familiar illustration of a doorslammer specification that typically utilizes a four-piston caliper in the rear and a two-piston caliper on the front.

The key ingredient to any brake system is line pressure. That pressure determines the clamping force of the calipers. We recommend at least 1,200 psi as the ultimate line pressure under severe braking conditions. – Andrew Dickson, Mark Williams Enterprises

In this case of six pistons between our four brake corners, both TBM and Mark Williams recommend a 1-inch master cylinder to provide the proper brake fluid volume based on their individual brake caliper designs. Extremes such as just a pair of rear calipers on a dragster or using 6-piston calipers all around in a road race car will dictate entirely different master cylinder specifications.

“Many racers will incorrectly say that their brakes aren’t working effectively so they need a bigger master cylinder for more flow,” Cotteleer explains. “A larger diameter master cylinder bore size will give a firmer pedal with less travel, but at the expense of lower line pressure and even less braking force. To change to a smaller piston master cylinder will increase line pressure but at the expense of longer pedal travel.”

Rear brakes for drag racing typically consist of a visually beefier four-piston design. Lightweight front brakes may use a larger diameter two-pistons caliper design that saves front end weight but actually has similar stopping power as the smaller diameter four-piston rear brake designs.

The overall theory between the master cylinder and brake pedal relationship is that a driver can either run out of leg strength or pedal travel to reach the desired 1,000 to 1,200 psi in brake pressure. The balance of these two requirements falls into calculating a proper brake pedal ratio.

“You’re trying to balance this lever called a brake pedal,” Dickson expands. “It has to be a wider ratio on each side of the pedal’s pivot point to convert your leg pressure to the needed pressure at the master cylinder. It also needs to be small enough of a ratio to not run out of travel to get to your needed pressure.”

By dividing the distance of the foot pad to the pivot point by the distance between the master cylinder rod clevis to the pivot point, you achieve your pedal ratio. Many original factory pedal ratios are the 6:1 range, meaning that 200 pounds of force on the pedal equals 1,200 pounds at the master cylinder. That ratio is much lower on the typical tube chassis drag car pedal. For a 4.5:1 ratio, for example, it would take 266 pounds at the pedal to achieve the same force at the master cylinder.

Visiting the Mark Williams website will show you diagrams and a brake system pressure calculator that can provide the optimum pedal or brake handle ratio based on driver’s force and master cylinder size.

With all of your brake system in place, we have met our goal of 1,000 to 1,200 psi of brake line pressure, but how do we know that? Our experts talk about the single tool that provides those answers and diagnoses any future problems, all for the price that equals many other general tools in your toolbox.

All of our brake pressure, volume, and related theories rely on utilizing a brake pressure gauge. You would never have a racing engine without an oil pressure gauge. –  Randy Cotteleer, TBM Brakes

“Racers many times will ask about brakes that won’t stop, are less effective than before, or they’re really grabby,” Cotteleer says. “I ask what their brake pressure is, but they have no idea what you’re talking about.”

A brake pressure gauge is readily available and retails for $50 and up. It is typically a high-pressure gauge that measures brake pressure at the caliper from 0-1,500 psi. It directly threads into the brake bleed port to give you a reading of clamping force. This is the absolute measurement to achieve that ultimate 1,200 psi brake pressure force.

“Pressure is pressure when it comes to a hydraulic system,” Dickson expresses. “The pressure does not split between the four corners of a brake system. This makes a pressure gauge an effective tool at not only measuring your effective pressure, but also diagnosing all four corners for blockages, front and rear balance, master cylinder problems, and more.”

Compared to a factory brake system, front and rear brake balance needs to be adjustable between big slicks, skinny front tires, and drastically different weight balances. TBM Brakes designs Tilton proportioning valves into their systems while Mark Williams features their own line of valves. Combination proportioning valves are available that mock an original manufacturer valve, but allows the needed adjustability to the rear brake circuit.

Another component usually incorporated into the entire brake system is the use of residual valves. These small valves are mounted along the brake line circuit and can serve two purposes. The first purpose is to maintain a small amount of pressure to the brake caliper, typically to keep a slight 2-pounds of pressure on the caliper.

This small amount of pressure keeps caliper pistons from completely retracting into their bores. Some racers discount the residual valve holding their brakes as free from drag as possible. This valve is something to research with your specific brake supplier.

If your brake pistons are designed to retract when you’re not applying the brakes, a residual valve can prevent that motion from happening and lessen the travel in your pedal when you start applying brake pressure.

Drivers can significantly vary in the amount of pressure that can apply to the pedal. Calculating your pedal ratio is the starting point between your leg strength and proper brake pressures. The Mark Williams Enterprises website offers a calculator to convert your foot pressure to the pressure at the master cylinder piston.

The second use of the residual valve is for race car chassis designs that incorporate the master cylinder along the floorboard. With the brake pedal pivot along the floor line, the master cylinder is located below the height of the brake calipers. The residual valve, in this case, prevents gravity from returning all brake fluid back to the master cylinder reservoir.

These residual valves should be mounted in the brake line system as close to the master cylinder as possible.

There’s a common saying that, “It’s not the brakes that stop a car, but the tires.” That comment couldn’t be more accurate when applied with the front to rear brake balance on a drag car that uses drastically different drag slicks and skinny fronts. The final piece of hardware to discuss in our brake pressure tech is an adjustable brake pressure proportioning valve.

A brake pressure gauge is the single most effective tool at building or troubleshooting your brake system. For the same price of many common tools in your shop, you can measure brake pressure at each bleeding orifice at each brake caliper.

“When we recommend a drag racing brake system, we typically don’t offer a four-piston caliper on the front,” Cotteleer illustrates. “We spec a two-piston caliper on the front and a four-piston caliper on the rear. This combination naturally proportions things a little better. We will recommend using a proportioning valve in the back because typically you’ll want to kill some of the pressure in the rear for balance.”

Dickson expands on the application of proportioning front and rear brake balance, noting Mark Williams’ approach to front brake proportioning. “If you’re running a full-size, four-piston caliper up front, typically we recommend the proportioning valve in the front to kill some of the front pressure as to not overpower the front tires.”

“Proportioning can also be significantly different between a lightweight altered with four brakes and a much heavier bracket car,” Dickson finishes. “Tires and weight characteristics make a proportioning valve invaluable. You don’t want the front or rear locking up in a hard-braking situation.”

Any overpowering of either the front or rear tires create an extreme demand on one or the other. By adjusting an equal amount of balance between all your brakes, you have not only a safer situation but also brakes that will perform for a much longer time.

Brake pressure is also a topic of discussion for the starting line as well. Many racers will incorporate brake pressure gauges into their race car dashboard to determine a proper and repeatable brake pressure.

Both TBM Brakes and Mark Williams Enterprises offer their own dual line, aluminum case master cylinder design with a plastic dual reservoir. DOT 5.1 brake fluid is specified by both professionals interviewed; TBM offers their own Xtreme line of fluid that provides a 612-degree Fahrenheit dry boiling point to prevent brake fade.

Good brake control is essential to hold the racecar correctly in the water box or on the starting line against the engine and torque converter while “footbraking.” Racers using clutches also want to keep the car in the staging beams before launching. All these scenarios require less brake pressure than top end braking, but clean brake pressure and pressure release is still critical.

When considering brake pressure, you need to consider the type of brake fluid you use, as well. Both Mark Williams Enterprises and TBM Brakes specify the use of DOT 5.1 brake fluid. Though Mark Williams simply recommends the use of these brake fluids that earn this 5.1 certification, TBM not only suggests the fluid spec in general but also offers their own DOT 5.1 product.

Compared to more typical DOT 3, 4, and even DOT 5.0 silicone fluids, the hygroscopic content of DOT 5.1 fluid has a 612-degree dry boiling point. This fluid is proven to work well in a drag racing application.

TBM offers these custom residual valves with male or female and NPT or AN fitting options. This eliminates unnecessary fitting adapters. These valves eliminate excessive pedal travel and prevent fluid from overflowing a floor-mounted master cylinder (arrow) where the cylinder is mounted lower than the brake calipers.

When heat builds in the brakes, lower grade brake fluid with a lower boiling point can boil from caliper heat. This boiling liquid produces gas bubbles. As with any boiling liquid, these gas bubbles will compress within the brake fluid and cause absorbed pressure and brake fade.

Both experts also hit upon brake pressure related to deflection when it comes to the clamping force of the brake pistons onto the brake disc rotors.

If the caliper and rotor are not aligned with a high degree of accuracy, the deflection can use up brake pressure to bring together the misalignment. The pedal then feels spongy, and racers go crazy trying to blame bleeding problems. – Andrew Dickson

“A good caliper by design means everything,” Cotteleer adds. “The primary failure for a weak caliper is that they tend to open like a taco. The bottom contact area spreads from the pressure. That uneven contact area can cause even more heat problems with the brakes. Watch your brake pads closely for any taper. Any defection, uneven clamping, or misalignment can ultimately be felt back in the brake pedal.”

When hitting the finish line, your brake design, pressures and balance can be your best friend or worst enemy. Fighting a weak brake system can impose unnecessary heat and wear that can exacerbate even more significant problems for you in the shutdown area.

Article Sources

About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
Read My Articles

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