Saturday, July 21, 2018 Detailed Auto Topics
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Pressing the brake pedal results in stopping the vehicle. The "feel" of the pedal is also important. When a brake pedal starts to travel too far before stopping, there is a problem.

Excessive brake pedal travel indicates a problem

By design, brake pedal height and travel differ slightly from one vehicle to another. Something is wrong, when the height or travel of the brake pedal changes. Diagnosing a low brake pedal requires an understanding of the principles involved. It appears very simple, but many things need to be considered with brake operation. To understand pedal height, mechanical and hydraulic multiplication of force must be considered.

Mechanical multiplication of force

A brake pedal is a class two lever, increasing force at the expense of distance

The force required to engage the brakes is greatly reduced by mechanical advantage. The brake pedal is a class-two lever. In the drawing above, the lever offers a three to one advantage. Force applied to the pedal is multiplied three times in the example above. 

With physics, when one thing is gained, something else is traded. The increased force comes at the expense of greater travel. This means our foot must move three-times the distance that we move the master cylinder push rod. This reduces brake effort, but increases pedal travel. Greater leverage results in more travel. To achieve a high brake pedal, movement of brake pads and shoes must be kept to a minimum. Any additional clearance between brake pads and the braking surfaces will result in increased pedal travel or a lower brake pedal.

Hydraulic multiplication of force

Simple hydraulic multiplication of force

Hydraulics are used to transfer force and to multiply the force applied. Increased force is based on the area of the pistons in the system. In the drawing above, the piston of a larger diameter is assumed to be twice as large as the smaller piston. Like our lever above, the bigger piston moves half the distance of the small, but with twice the force.  This is hydraulic multiplication of force and is used to increase the clamping force of calipers and wheel cylinders.

Hydraulic advantage is used by the brake master cylinder. The area of the piston in the master cylinder is less than that of the brake caliper. This reduces pedal effort but increases travel. The greater the difference in the piston sizes, the more multiplication of force and the increase in travel. Since a single piston is moving two caliper pistons, pedal travel in doubled.

Engineers go to great lengths to keep brake pad and shoe movement to a minimum. Anything that increases movement, even slightly, will result in a lower brake pedal.

Tests to find causes of a low brake pedal

A few simple tools and an understanding of the system are required for finding the cause of a low brake pedal. The first step is to isolate if the problem is with a wheel or the master cylinder.

Being able to block off parts of the hydraulic system allows an easy method of finding the problem cause. Great care must be exercised, particularly with older brake systems. Brake hoses that have become hard will be damaged by clamping. Inspect all hoses and replace any that appears deteriorated, dry rotted or brittle.

Tools to block off brake hoses for diagnosis

Special tools are available for temporarily blocking brake hoses, as a diagnostic procedure. There are several types and most work adequately. Vice-grips should NOT be used, as these may damage the hoses. Three or four tools will be needed, depending on if there are one or two rear hoses.

Understanding the tests for a Low Brake Pedal

Temporarily block all brake hoses with the proper tools. Brake fluid cannot flow to the wheels, so they are currently taken out of the system. If pedal height is restored, the problem of low pedal is with one of the wheels. This simple test eliminates the master cylinder and ABS unit as possible causes. A pedal that remains low, with all wheels blocked, means there is a problem with the master cylinder, ABS hydraulic module or air in the upper system.

Air in a brake system will cause the pedal to be spongy and low

Hydraulic fluid does not compress so it transfers force applied to it. When the fluid is drained, gets low or leaks out, air can enter the system. Air is compressible and acts like a spring. Any air trapped in the system will increase fluid movement and drop the brake pedal.

Air is removed by ‘bleeding’ it from the system. Bleeding procedures vary widely on modern vehicles. Using an improper method or sequence will result in more air being drawn in. Service data for the vehicle being repaired must be consulted to learn the proper sequence and procedure. Many newer vehicles require a factory-equivalent scan tool to bleed air from the system. For the enthusiast, the best plan is to avoid air entering the system, by keeping a close watch on the fluid level during service. In the next article in this series, air entering the system is covered in more depth.

Brake problems at the wheels

If the pedal is firm with the wheels blocked off, the tools should be removed, one at a time. As each tool is removed, the pedal should again be tested. A sudden drop in pedal height, when a wheel is returned to the system, indicates the problem wheel. An equal amount of a drop at all wheels, shows the problem to exist throughout the system. A likely cause would be air in the system.

Master cylinder and ABS module problems

Sometimes, blocking off all wheels will not return the brake pedal to normal. Here, there is a problem in the upper system, including the master cylinder and ABS control module. Each of these components will cause the same symptom. Isolating the ABS module is complex and best left to a professional.

The brake lines to the master cylinder will be either steel or braided steel. These cannot be pinched off for testing. Instead, a thorough inspection of the master cylinder for signs of leakage may help. Remove the cylinder from the brake booster, and make sure the fluid is not leaking out of the rear. Any leakage means the master cylinder must be replaced.

Plastic hoses and fittings used to bled a brake master cylinder

When replacing the master cylinder, it must be independently bled before attaching the lines. Failure to remove the air from this component, will result in it being pushed into the system. Always consult service data before attempting to replace such a component yourself. This can avoid having to tow the vehicle to a shop to bleed a system that requires special tooling.

In the next article, finding the exact cause and further diagnosis of a low brake pedal will be discussed.





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