When upgrading a performance vehicle's original drum/drum or disc/drum braking system to disc/drum or four-wheel-disc brakes, several variables must be considered when determining correct master cylinder size. These variables include vehicle weight, the weight bias of the vehicle, the tire and wheel setup, brake configuration (disc/drum, disc/disc, etc.), brake disc diameter, caliper design and location, caliper piston sizes, brake pedal ratio and planned driving application (track or street).
A master cylinder designed for use with drum brakes will move less fluid volume than one designed for use with disc brakes. If you're changing a drum brake setup to disc, you'll need a master cylinder with a larger bore diameter. In many cases, a drum brake setup will feature a 10-lb. residual valve at the outlet to maintain a slight residual pressure on the drum brakes. If connected to disc brakes, the drum master (with residual valve) will drag because of the residual pressure. If you use a drum brake master cylinder that doesn't feature a residual valve, the brake pedal will be spongy, requiring pumping of the brakes to attain sufficient pressure.
A dual master cylinder designed to work with a disc/drum setup will feature two different reservoir sizes. As disc pads wear, the caliper pistons will move further outward toward the rotor, causing the reservoir fluid level to drop more than it would for a drum brake reservoir. As a result, a disc/drum master cylinder needs a larger reservoir for the disc brake circuit to keep more fluid in reserve. If switching from a disc/drum setup to a four-wheel-disc setup, the original disc/drum master cylinder won't be suitable, since a four-wheel-disc master requires a longer piston stroke.
A drum/drum master cylinder delivers equal volume and pressure to the front and rear brakes. It typically will be smaller than a disc master, and fluid reservoirs will be the same size. Drum brake master cylinders require residual valves. Original drum master cylinders may feature built-in residual valves. Aftermarket master cylinders may require residual valves to be added to the plumbing circuits.
A disc/drum master cylinder requires more fluid volume for the disc brake circuit, so the disc reservoir will be larger. An original disc/drum master cylinder may feature a built-in residual valve for the drum brake circuit. If a replacement disc/drum master cylinder is selected, an external residual valve may need to be added.
A disc/disc master cylinder (four-wheel-disc) features sufficient volume and stroke to provide required fluid volume to all four calipers. If a disc/drum master cylinder is used on a disc/disc system, the master cylinder will run out of stroke while trying to push fluid to the rear brakes, resulting in a long pedal travel and a spongy pedal feel.
If the brake system is manual (no power booster), the master cylinder bore diameter should be less than 1 in., otherwise a very hard pedal will result. A power brake master cylinder will typically feature a larger bore diameter than a manual master cylinder. A manual master cylinder can be mounted to a power booster, but a power master cylinder likely cannot be used without a power booster, because its larger bore size will result in a hard pedal, and the piston hole will be too shallow, which may allow the pushrod to fall out of the piston's pushrod hole. A master cylinder's rear pushrod hole must be deep in order to keep the pushrod in place.
If a performance vehicle currently is equipped with front disc and rear drum brakes and the customer wishes to upgrade to rear disc brakes, the changeover will require replacing the current master cylinder with a true four-wheel-disc master cylinder, since the change to disc from drum will require additional fluid volume. Generally, drum brakes require about 400 to 500 psi, discs 800 to 900 psi. The new master cylinder (for four-wheel disc brakes) will require a longer stroke and larger reservoir for the rear circuit.
Upgrading to rear discs also requires changing the combination valve to one designed for the application and/or adapting the parking brake cables to the rear disc system, in addition to properly adjusting the rear calipers.
Unless the customer is determined to have rear disc brakes primarily for "bragging rights," and if the stopping power with the current front disc/rear drum setup is adequate, try talking him out of the change to rear discs. Rear disc brakes aren't always needed. Especially in cases where the vehicle is equipped with custom wheels that readily reveal or display the rear brakes, many customers will opt for rear discs merely for show, even if braking force performance is already satisfactory. Often, the change from rear drum to rear disc brakes is more trouble and expense than it's worth.
There are some things you must know when selecting a master cylinder. A larger-diameter master cylinder will displace more fluid volume than a smaller-diameter unit, but will create less line pressure. The result is a harder pedal requiring more braking effort. Also, whenever possible, be sure brake fluid level is above both the master cylinder and the brake calipers, to prevent bleed-down. If a residual pressure valve is needed in the system, use a 2- to 4-lb. valve for disc brake applications.
When you get right down to it, master cylinder selection is a trial-and-error process. Pedal feel and what's comfortable for the individual driver will dictate the final choices.
Whenever possible, always try to mount the master cylinder as high in the vehicle as possible. If a remote fluid reservoir is used, mount it as high as possible as well. The object is to make sure all lines run at a downward angle from the master to the calipers.
It's always best to mount the brake fluid reservoir and master cylinder higher than the calipers, to prevent brake fluid from draining back to the master reservoir, which can deplete the caliper fluid, causing the driver to pump the heck out of the pedal to push the fluid back to the calipers. When this mounting layout isn't possible&#;such as when the master cylinder is mounted on the vehicle frame (below the floor)&#;the use of a residual valve is the answer. It prevents brake fluid from backing up into the master cylinder reservoir by applying a bit of residual pressure in the circuit.
A residual pressure valve is a small valve installed in the brake line in certain applications. It retains a minimum brake line pressure to help eliminate excessive pedal travel in both disc and drum systems. A 2-lb. valve is used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers, where fluid drainback results from gravity and vibration. This causes excessive caliper piston retraction, and therefore a longer pedal stroke to regain the pedal.
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The 2-lb. valve prevents fluid from flowing back to the master cylinder without causing the brakes to drag. It maintains 2 lbs. of line pressure when the pedal is released. A drum brake setup requires a 10-lb. valve to compensate for the return spring tension in the drums. Don't use one if you don't have to, since this slight amount of line pressure is always present, with the potential to cause the brakes to drag if the valve has a problem. However, if the master is mounted lower than the calipers, you don't have a choice. Use it, but mount the valve where you can inspect it and easily replace it if necessary. The best place to mount a residual valve is at the end of the master cylinder, where it's obvious and can be seen and accessed easily.
Air inside the hydraulic system causes a mushy pedal (or loss of pedal, depending on the amount of air in the system). Remember, you can compress air, but you can&#;t compress fluid. The object is to use a pure fluid transmission from the master to the caliper pistons, which will operate the pistons immediately with no lag time that would be caused as you try to compress trapped air.
The brakes should be bled before every single race. Don&#;t assume that just because they were bled in the shop in April that they don&#;t have to be serviced throughout the season. A regular schedule of bleeding provides an opportunity to remove any air that has been sucked into the system during use, and it offers a perfect chance to freshen the brake fluid. Brake fluid is hygroscopic, which means it sucks up water like a magnet grabs grinding dust from your workbench. As little as 3% moisture content can cut your brake fluid&#;s boiling point in half! Even the best and most expensive brake fluid will not last forever.
There are several methods of bleeding. You can gravity-bleed by opening all of the caliper bleed valves and letting them drain, followed by manual pumping/bleeding; you can use a handheld vacuum bleeder tool; or you can use a pressure bleeder. Gravity bleeding and manual pumping can be timeconsuming and messy. Vacuum bleeders are iffy at best, since some vacuum bleeder tools frequently leak or fail, and their hoses sometimes slip off the nipples, creating a mess and allowing air to reenter the system. The best approach is to use a pressure bleeder.
Connect the pressure bleeder hose to the adapter fitting on the vehicle&#;s brake fluid reservoir. Once you&#;ve installed the appropriate adapter to the fluid reservoir, set the bleeder pressure to about 30 psi. Connect clear hose to each caliper&#;s bleed valve and insert the other end of the hose into a clean container partially filled with fresh fluid. Open the pressure valve on the pressure bleeder, and open the caliper bleed valve until you see clean fresh fluid with no air bubbles, then close the caliper bleed valve.
Always start with the caliper farthest away from the master cylinder and work your way toward the master. This typically will involve bleeding the right rear first, followed by the left rear, right front and finally the left front. When using a pressure bleeder, do activate the brake pedal.
If you opt to perform a manual bleed, work on one caliper at a time, again starting at the caliper that&#;s farthest away from the master cylinder. Connect a clear hose from the caliper bleed valve and insert the other end of the hose into a clean partially filled container (this will prevent air from being sucked back into the system). While a helper pushes down and holds the brake pedal, open the caliper bleed valve to allow fluid to escape, then close the bleed valve. Once the valve is closed, the helper can release the brake pedal. Wait 3 to 5 seconds and repeat the process. Continue repeating the entire process until clean, airless fluid is seen leaving the caliper. Avoid pumping the pedal, as this will only aerate the fluid and make bleeding more difficult. Perform the entire bleeding process at all caliper locations at least twice (once all the way around the vehicle, followed by another complete pass around the vehicle).
Make sure someone pays attention to the brake fluid reservoir to keep it topped with fresh fluid. Never allow the fluid level to sink lower than 1 &#;2 in. above the reservoir floor.
After bleeding the system (regardless of the bleeding method), always check for free rotation of all rotors. If a rotor suddenly begins to drag on the pads, stop and diagnose the problem.
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Carl Bush: Although racing with a perfectly centered balance bar is the ideal goal, it seldom happens in reality. Besides, one of the advantages in using an adjustable balance bar is having the ability to adjust that leverage to optimize handling and driver comfort on track. Trying to measure the post-race leverage split at the balance bar is difficult and unrealistic. However, using pressure gauges to measure pressure differentials s at any given balance bar setting is relatively simple. The brake gauges will show the actual pressure split in the car based on the balance bar adjustments made by the driver. Those pressures can then be multiplied by the effective caliper piston bore areas to calculate the last on-track static bias settings.
Going back to our (common setup) example, if we apply 50 pounds of leg force against a 6:1 pedal, we will generate 300 total pounds of force against the balance bar. If the balance bar is perfectly centered, it will distribute that force equally to each master cylinder. With each master cylinder receiving an equal force amount of 150 pounds, the 7/8&#; master cylinder should produce 250 PSI (Jeff&#;s math: 250 PSI comes from 150 divided by .6 which is the 7/8&#; master cylinder math result) while the rear 1&#; master cylinder produces 192 PSI (Jeff&#;s math: 192 PSI comes from 150 divided by .785 which is the 1&#; master cylinder math result). In practical use of gauges, you can use any level of effort and pressure for your comparisons. The end result will be the same.
When the front pressure of 250 PSI from the 7/8&#; master cylinder is multiplied by the 4.8&#; inches of caliper bore area of the front 1.75&#; piston front calipers, we get a front clamping force of . On the rear, we will have 192 PSI x 2.97&#; caliper area or 570 pounds of rear caliper clamping force. When comparing the these front to rear clamping force total in the same way you would compare wheel weights for balance, we would see that this car has a total of pounds of caliper clamping force at these line pressures with pounds or 67.8 % on the front. It&#;s that same static bias ratio that was measured using the overall driver leverage ratios.
Now, if every car and driver had the same braking requirements and pedal feel preferences, we would never need to adjust anything. But, every car and every driver are unique and adjustments will get made.
The ratio examples that have been used here are very common in many short track asphalt cars. But your car, for a wide variety of reasons, may have quite different requirements. As a racer or crew chief, you can use these formulas to map the existing brake setup on your own race car, and then make calculated decisions when the desired handling or driver feel isn&#;t being delivered. The inability to reach the desired bias or driver&#;s feel of the pedal is the indication you will need to evaluate your component selection and consider possible alternatives. By using the formulas in these examples, you can accurately calculate what affects a component change will make to your existing baseline, and record those final ratios in your records to use for future adjustments and set up for any given track type or conditions.
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