Using the Right Master Cylinder
Master cylinders come in a variety of sizes, capacities, mounting configurations, plumbing configurations, and some have valves built into them.
Reservoir Size
The master cylinder reservoir should always hold enough fluid to allow the brake pads to completely wear away without the risk of running out of fluid. As a disc brake pad wears, the caliper piston will extend out of its bore. As the caliper piston extends, the master cylinder fluid level will drop in order to fill the caliper bore with brake fluid. If the reservoir is too small, there is a possibility that the master cylinder can run out of fluid and the brakes will fail.
Mounting Configuration
There are two common mounting configurations. One is known as deep bore and the other is known as shallow bore. This refers to the way the pushrod engages the master cylinder. A deep bore master cylinder will have the pushrod extend about 1-1/2" into the master cylinder. A shallow bore master cylinder will have a pushrod extend about 1/4" into the master cylinder. All manual brake applications should be a deep bore set up. In a deep bore setup, the pushrod will go about 2" into the master cylinder. This deep engagement ensures that the pushrod can not accidentally fall out of the master cylinder.
Some power booster applications require a deep bore master cylinder; however, most use a shallow bore master cylinder. The relation of the pushrod to the master cylinder is very important. If there is too much clearance between them, the brake pedal will be low and may go all the way to the floor before the brakes start to function. If there is not enough clearance, the master cylinder may become “preloaded” by the pushrod. If the master cylinder has preload, the brakes will drag and get hot; when this happens to a disc brake, the brakes can sometimes lock up. It may become impossible to move the vehicle until the brakes have cooled down or the bleed screws are opened to relieve the hydraulic pressure.
Plumbing Configurations
Most vehicles built before 1967 were equipped with a single-circuit master cylinder. These master cylinders are also known as a single system. If any portion of a single system were to fail, there would be no brakes. A single circuit master cylinder can be easily identified by the single tube plumbed into the master cylinder. Every vehicle made from 1967 on will have been equipped with a dual circuit master cylinder. These have two independent hydraulic systems working inside one master cylinder. If one portion of the system failed, the second system will continue to function. These dual systems will have at least two tubes plumbed into the master cylinder. Some of these dual circuit master cylinders will have provisions to plumb four tubes into them. This can be helpful when using the master cylinder in a custom application where there is not enough space on one side of the master cylinder to plumb the tubes.
Master Cylinders with Built-in Valves
Some master cylinders have valves built in. These valves work similarly to the valves that would be plumbed externally. Internal valves have fewer connections and, therefore, fewer places for a potential leak. They simplify the plumbing and help make the system look better.
Bolting the Parts Together
The entire brake system should be dry-fitted and adjusted before any brake fluid is added or the system is bled. This is done so that you can remove part of the system in order to make an adjustment and not loose any fluid or have to re-bleed the brakes.
Pedal Assembly
Something that many people are not aware of when upgrading their system is the brake pedal adjustment. Improper pedal adjustment can do the following:
• Preload the booster, causing brakes to drag and eventually lock up.
• Damage the booster's internal components by actuating at an improper angle.
Many vehicles have a second hole on the pedal assembly that was originally intended for power brake applications. The required hole is generally 1" to 1-1/2" lower than the original manual brake hole. If there is no second hole, you may need to drill the hole in the pedal arm to properly align the pushrod. The easiest way to determine the perfect location for this hole is to find the center of the pushrod's vertical travel to ensure that its actuation is as inline with the booster as possible.
1. Gently lift the end of the pushrod until it stops, and note the location where the clevis on the pushrod locates on the pedal arm. You can mark this location.
2. Then gently push the pushrod down until it stops. Mark this location as well.
3. The correct location for the hole is in the middle between the two marks on the pedal arm. Mark and drill the new hole in the center of the pedal arm. Attach the pushrod to the pedal arm.
Pedal Rod Adjustment
Check free play in the pedal by applying pressure to the pedal with your hand and noting how far the pedal travels before resistance is felt. It should be approximately 1/4"; adjust the rod as needed. This free play allows the master cylinder piston to return to the "at-rest" position and prevents preloading the system. Too much free play will drop the pedal too far before applying brake pressure and the pedal may even hit the floor before applying full braking force to the system. Free play is also dependent upon proper adjustment of the booster pushrod. See "Adjusting the Booster Pushrod" below. Note: It is a good idea to paint the master cylinder to prevent rust.
