How Does a Tandem Master Cylinder Work?

as published in British V8 Newsletter, Volume XIV Issue 3, December 2006

by: Curtis Jacobson

(Editor's Note: two articles have been submitted to this issue of The British V8 Newsletter that address performance modification of the MGB brake system. This article was originally conceived as a "sidebar" article to provide general background information to support these two articles. It should answer the basic question: "How does a tandem master cylinder work?" Over time, this article has grown in scope to include a few suggestions a person might also consider when evaluating the frequently asked question: "Should I upgrade my brake system?")

One of the most frequent questions asked by people contemplating an engine swap for their classic British sports car is "Should I upgrade the brake system?" It can be a difficult question. Most of these cars had excellent brakes by the standard of their day. Since the popular V8 and V6 engine alternatives are similar in weight to the original British engines, the actual need for stronger brakes isn't always clear. (If the brakes are strong enough that you can lock them up at will, you might be wise to spend your money on stickier tires before modifying the brakes.) However, several other factors may indicate a need for brake system redesign. First and foremost, most cars produced before 1968 will have single-piston master cylinders. A leak in brake plumbing anywhere on the car can render these brakes completely inoperable. If your car falls in this category, upgrading to a later model tandem master cylinder is strongly recommended.

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A tandem master cylinder is characterized by two pistons operating in series within a common bore, as shown in the two illustrations below. In rear-wheel drive applications the piston that's located closer to the pedal (labeled "Piston 1") applies the vehicle's front brakes. In normal operation, fluid displaced and pressurized by Piston 1 also causes movement of a second piston ("Piston 2"). Piston 2 applies the vehicle's rear brakes.

The following two illustrations show how a tandem master cylinder isolates leaks in the front and rear brake plumbing respectively. (In both illustrations, the pedal has already been depressed to the point of brake application.)

How does a Tandem Master Cylinder work?

As shown in Illustration 1, if a leak develops in the front brake system, Piston 1 will move forward until it contacts Piston 2. Force from the brake pedal will be transmitted mechanically through Piston 1 to Piston 2. Although overall braking performance will be severely compromised, the rear brakes will still be functional provided sufficient pedal travel is available. The pedal will need to travel further than normal to fully engage the rear brakes. Also, it should be appreciated that trying to stop quickly with just the rear brakes is very tricky because the rear tires will easily reach the point of lock-up. As the car is slowing, weight transfers forward and the rear wheels lose some of their much needed traction.

Brake master cylinder design

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If a leak develops in the rear brake system, Piston 2 will move forward until it contacts the closed end of the master cylinder housing. Once Piston 2 becomes stationary, pressurization of fluid between the two pistons will apply the front brakes. Although overall braking performance will be significantly compromised, the front brakes will still be functional provided sufficient pedal travel is available. The pedal will need to travel further than normal to fully engage the front brakes. (Frankly, some inattentive or inexperienced drivers have been known to continue driving with non-functional rear brakes, despite longer pedal travel and longer stopping distances. For this reason, newer cars are fitted with brake failure warning lights.)

For purposes of illustration, return springs have been omitted from the two illustrations above. There are typically two coil springs in series: one acting between Piston 1 and Piston 2, and the other acting between Piston 2 and the closed end of the master cylinder housing. Although Piston 2 is shown fully forward in the second illustration, it is only held there (temporarily) by fluid pressure between the pistons. It might be of further interest to note that the illustrations above apply to most rear wheel drive cars produced since 1968 - but not to front wheel drive cars. Front wheel drive cars are normally plumbed differently, such that each piston applies one front and one rear brake cylinder (on opposite sides of the car.)

How does one go about updating a "single line" brake system? If your model of car was produced both before and after tandem master cylinders were mandated, it will probably be easiest to simply purchase and install later model components. Otherwise, you'll have to do some research... Generally, in selecting a tandem master cylinder there are several specifications of interest. One of the first to consider is bore size. A larger diameter piston will displace more fluid per unit of travel. Less pedal travel will be required for a corresponding amount of braking force at the wheels. Pedal effort will also be greater for a corresponding braking effect.

Another issue when selecting a tandem master cylinder is whether it comes with one or more internal residual pressure valves. On an OEM master cylinder designed for a car with rear drum brakes there will sometimes be a residual pressure valve in the rear brake circuit to maintain a small amount of pressure at the wheel cylinder seals. The amount of residual pressure varies from one design to another but typically is between 6 and 25psi. Disc brake calipers may or may not need a residual pressure valve depending on their design. A 2psi residual pressure valve is often recommended for aftermarket disc brake calipers.

What other factors would influence an engine swapper's decision of whether to undergo a partial or complete redesign of his car's braking system?

Many people who complete an engine swap also swap rear axles, and not necessarily because the original axle is perceived to be too weak. A rear axle swap is often the way to go for anyone who wants a limited slip differential and/or a different (usually higher) gear ratio. However, if the rear axle comes with its own brakes, the braking system may need to be re-evaluated as a whole to assure proper front-to-rear balance.

Additionally, many engine swappers will want to increase tire width and/or diameter. With modern radial tires, and especially with wider contact patches, quicker stopping becomes quite feasible - but modification of the brake system may be necessary to take full advantage of the possibilities. Larger tires and wheels are more effective flywheels. (Even if they don't weigh more than the original wheels and tires, their weight is distributed further from their axis which makes them more effective at storing energy.) At high speed the momentum stored in the rotating wheels becomes significant. It's quite possible that stopping distances will actually increase significantly with up-sized tires and wheels if the brakes aren't also upgraded. Jeff Schlemmer's article "Ultimate $15 Brake Upgrade" describes one clever approach to dealing with this issue. A second and somewhat more conventional approach is to fit "big brakes". Larger diameter rotors can provide significantly more braking torque, whereas vented rotors are able to shed more heat and thus resist "fading" during hard use.

Disclaimer: This page was researched and written by Curtis Jacobson. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk. For further information we recommend "Brake Handbook" by Fred Puhn.

Illustrations by Curtis Jacobson. Copyright 2006. All rights reserved.

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