Master Cylinder

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. hen upgrading a performance ve- will move less fluid volume than one designed for use hicle’s original drum/drum or with disc brakes. If you’re changing a drum brake setup disc/drum braking system to to disc, you’ll need a master cylinder with a larger bore disc/drum or four-wheel-disc diameter. In many cases, a drum brake setup will feature brakes, several variables must be a 10-lb. residual valve at the outlet to maintain a slight considered when determining cor- residual pressure on the drum brakes. If connected to rect master cylinder size. These variables include vehicle disc brakes, the drum master (with residual valve) will weight,W the weight bias of the vehicle, the tire and wheel drag because of the residual pressure. If you use a drum setup, brake configuration (disc/drum, disc/disc, etc.), brake master cylinder that doesn’t feature a residual brake disc diameter, caliper design and location, caliper valve, the brake pedal will be spongy, requiring pumping piston sizes, brake pedal ratio and planned driving appli- of the brakes to attain sufficient pressure. cation (track or street). A dual master cylinder designed to work with a A master cylinder designed for use with drum brakes 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 re- serve. 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. Photo courtesy Wilwood Engineering courtesy Photo A disc/drum master cylinder requires more fluid vol- Aftermarket performance master cylinders are avail- ume for the disc brake circuit, so the disc reservoir will be able in a variety of configurations. Shown here is a tan- larger. An original disc/drum master cylinder may feature dem polished cast aluminum unit sporting a black an- a built-in residual valve for the drum brake circuit. If a odized lid. In other words, your customer has options replacement disc/drum master cylinder is selected, an ex- beyond OE-looking cast-iron or aluminum units. ternal residual valve may need to be added. 18 October 2012 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, re- sulting 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 Photos: Jim Gibson Photos: will typically feature a larger bore di- 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 up- ameter than a manual master cylin- graded this Plymouth Duster with disc brakes, we chose a cast-iron master der. A manual master cylinder can be cylinder that looks stock. A layer of clearcoat keeps it looking new. 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 Visit us at be too shallow, which may allow the AAPEX 2626. 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 www.vdo.com/usa changeover will require replacing the current master cylinder with a true VDO: coverage, quality and peace of mind four-wheel-disc master cylinder, since When you choose VDO, you can be confident you’ve made the right the change to disc from drum will re- call. VDO door system components are ready-to-install, right out of the quire additional fluid volume. General- box, restoring original equipment function and performance. The right ly, drum brakes require about 400 to parts, the first time, every time. That’s peace of mind. That’s VDO. 500 psi, discs 800 to 900 psi. The new For more information, call: 800-569-5066 master cylinder (for four-wheel disc or email: [email protected] brakes) will require a longer stroke and Power Window Motors and Gear Kits | Power Window Regulators larger reservoir for the rear circuit. Power Window Motor and Regulator Assemblies | Door Lock Actuators 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 VDO - A Trademark of the Continental Corporation VDO - A Trademark Circle #20 20 October 2012 A common master cylinder location for some street rods is an in-the- frame mount. This places the master cylinder be- low the driver’s floor, with an access panel for fluid checking/service access. This requires the master cylinder to be re- verse-mounted, with a custom pedal arm. Whenever a master cylinder is mounted low, you need to install residual valves to prevent flu- CONGRATS id from backing up to the master and to maintain a slight amount of positive pressure to keep fluid in the caliper plumbing. properly adjusting the rear calipers. master cylinder selection is a trial- Unless the customer is determined and-error process. Pedal feel and to have rear disc brakes primarily for what’s comfortable for the individual TOP 20 “bragging rights,” and if the stopping driver will dictate the final choices. power with the current front disc/rear drum setup is adequate, try talking Master Cylinder Tool Award him out of the change to rear discs. Mounting & Rear disc brakes aren’t always need- Residual Valves ed. Especially in cases where the ve- Whenever possible, always try to WINNERS hicle is equipped with custom wheels mount the master cylinder as high in that readily reveal or display the rear the vehicle as possible. If a remote l brakes, many customers will opt for fluid reservoir is used, mount it as t rear discs merely for show, even if high as possible as well. The object is r braking force performance is already to make sure all lines run at a down- r satisfactory. Often, the change from ward angle from the master to the If you missed this year’s t rear drum to rear disc brakes is more calipers. winners, or would like more trouble and expense than it’s worth. It’s always best to mount the brake information, visit: fluid reservoir and master cylinder www.motormagazine.com Master Cylinder higher than the calipers, to prevent Selection Tips brake fluid from draining back to the If you are interested in sub- There are some things you must master reservoir, which can deplete mitting a product for next know when selecting a master cylin- the caliper fluid, causing the driver to year’s Top 20 Tool Awards, der. A larger-diameter master cylin- pump the heck out of the pedal to visit our “media kit” link and der will displace more fluid volume push the fluid back to the calipers. select “Top 20 Tools Entry than a smaller-diameter unit, but will When this mounting layout isn’t possi- Information.” create less line pressure. The result is ble—such as when the master cylinder a harder pedal requiring more brak- is mounted on the vehicle frame (be- ing effort. Also, whenever possible, low the floor)—the use of a residual be sure brake fluid level is above both valve is the answer. It prevents brake the master cylinder and the brake fluid from backing up into the master 1-800-4A-MOTOR calipers, to prevent bleed-down. If a cylinder reservoir by applying a bit of residual pressure valve is needed in the residual pressure in the circuit. system, use a 2- to 4-lb. valve for disc A residual pressure valve is a small brake applications. valve installed in the brake line in When you get right down to it, certain applications. It retains a mini- October 2012 21 WE ARE EVERYWHERE YOUR CUSTOMERS ARE. Find us: • Television • Online • Social Media • Mobile • Point of Sale Here’s an example of a master cylinder designed for remote-mounting of the reservoirs.

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