When upgrading a performance vehicle s

Transcription

1 MASTER CYLINDER SELECTION BY JIM GIBSON Many options are available to performance vehicle owners wishing to upgrade their brake systems. Safe and effective operation depends on matching all parts, including the master cylinder. 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 Aftermarket performance master cylinders are available in a variety of configurations. Shown here is a tandem polished cast aluminum unit sporting a black anodized lid. In other words, your customer has options beyond OE-looking cast-iron or aluminum units. Photo courtesy Wilwood Engineering 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 fourwheel-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. 18 October 2012

2 In order to maintain a stock appearance during a restoration, you ll need to choose a master cylinder that offers an OE appearance. Even though we upgraded this Plymouth Duster with disc brakes, we chose a cast-iron master cylinder that looks stock. A layer of clearcoat keeps it looking new. VDO - A Trademark of the Continental Corporation Visit us at AAPEX VDO: coverage, quality and peace of mind When you choose VDO, you can be confident you ve made the right call. VDO door system components are ready-to-install, right out of the box, restoring original equipment function and performance. The right parts, the first time, every time. That s peace of mind. That s VDO. For more information, call: or salessupport-us@vdo.com Power Window Motors and Gear Kits Power Window Regulators Power Window Motor and Regulator Assemblies Door Lock Actuators Photos: Jim Gibson A disc/disc master cylinder (fourwheel-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. Power vs. Manual Brakes 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. Upgrading to Rear Discs 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 20 October 2012 Circle #20

3 A common master cylinder location for some street rods is an in-theframe mount. This places the master cylinder below the driver s floor, with an access panel for fluid checking/service access. This requires the master cylinder to be reverse-mounted, with a custom pedal arm. Whenever a master cylinder is mounted low, you need to install residual valves to prevent fluid from backing up to the master and to maintain a slight amount of positive pressure to keep fluid in the caliper plumbing. CONGRATS 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. Master Cylinder Selection Tips 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 trialand-error process. Pedal feel and what s comfortable for the individual driver will dictate the final choices. TOP 20 Tool Award WINNERS If you missed this year s winners, or would like more information, visit: If you are interested in submitting a product for next year s Top 20 Tool Awards, visit our media kit link and select Top 20 Tools Entry Information A-MOTOR Master Cylinder Mounting & Residual Valves 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 minil t r r t October

4 WE ARE EVERYWHERE YOUR CUSTOMERS ARE. Find us: Television Online Social Media Mobile Point of Sale Not just oil, Pennzoil..com 2012 SOPUS Products. All rights reserved. Circle #22 Here s an example of a master cylinder designed for remote-mounting of the reservoirs. Each reservoir may be mounted above the master, on the firewall. This design is handy for crowded underhood applications where access to direct-mount reservoirs on the master would be difficult. mum 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. 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. Brake Bleeding 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 22 October 2012

5 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. Sources for Performance Master Cylinders Alcon Components Outlaw Racing Disc Brakes AP Racing Baer Brakes, Inc. CNC, Inc. Master Power (MP Brakes) Stainless Steel Brakes The Brake Man Tilton Engineering Wilwood Engineering 24 October 2012