The Brake System:
Before Road-Test Inspection
The condition of the hydraulic service brakes of a vehicle can be determined by inspecting the following items: fluid level in the master cylinder, brake pedal free travel, total brake pedal travel, feel of brake pedal (hard or spongy), leaks in the hydraulic system, noise during operations, performance, and the amount of wear of brake parts. Wear can normally be determined by checking one wheel of each axle.
 To inspect the fluid level in the master cylinder, first clean away all dirt that may fall into the master cylinder reservoir. Remove the filler cap and ensure the fluid level is at the level recommended in the maintenance manual pertaining to the vehicle being serviced. The level of fluid is determined by measuring the distance from the top of the filler hole to the level of fluid in the reservoir.
Image right: Locate the master cylinder
If the fluid level is low, refer to the vehicle's lubrication order for the  recommended type of brake fluid and add fluid as needed. Anymore,most passenger vehicles use silicone brake fluid. Silicone fluid does not absorb water, provides good corrosion protection, and has good lubrication qualities. The fluid is also compatible with the rubber components of the brake system.
Check the master cylinder supply tank reservoir vent to make sure that it is not plugged. On some vehicles, a small hole drilled in the filler cap vents the supply tank. On other vehicles, the supply tank is vented through a line and fitting connected to the top of the master cylinder supply tank. A plugged vent can be easily cleared with compressed air.
Before adding brake fluid, make sure you are certain which type of brake fluid is needed. Adding the wrong type of brake fluid can result in damage and costly repairs.
Measure the brake pedal free travel and compare the measurement with the specifications given in the vehicle's maintenance manual. Brake pedal free travel is the amount that the brake pedal can be moved without moving the master cylinder piston. If the pedal has too much free travel, it will have to be pushed farther before  the brakes apply. If there is not enough free travel, it may prevent the brakes from releasing.
To check the total travel of the brake pedal, push the brake pedal down as far as you can. You should not be able to push the brake pedal on most trucks any closer to the floorboard than 2 inches.
- If there is too much pedal travel, but the pedal feels firm, the problem is probably caused by normal wear of the brake lining. When the lining is not worn too badly, an adjustment of the brake shoes will correct excessive pedal travel.  Unfortunately, the only way to determine the exact amount of the brake lining wear on most vehicles is to remove the wheels and brake drums.
Image right: Air in the brake line can reduce braking efficiency
- If the pedal travel is too great and the pedal feels spongy, there is probably some air in the hydraulic system. Air trapped in the hydraulic system can be compressed and does not permit pressure applied to the pedal to be applied solidly to the brakes. Methods of correcting these problems are covered later in this lesson.
 Inspect the hydraulic system for leaks. Large leaks can be detected while checking the pedal travel. This is done by holding a steady pressure on the brake pedal for a few moments. If the pedal continues to move down, there is a large leak. Small leaks cannot be detected this way as they cause the pedal to fall away too slowly to be noticed.
Look the entire hydraulic system over for any visible indications of leakage. Inspect the master cylinder, especially around the rubber boot, for external fluid leaks. Inspect all steel lines (tubes) for leakage, loose fittings, wear, dents, corrosion, and missing retaining clips. Inspect the flexible hoses for leakage, cuts, cracks, twists, and evidence of rubbing against other parts. Inspect the area at the lower edge of the backing plate for the presence of any brake fluid or grease. Leakage of either brake fluid or grease at the wheels is an indication of brake problems. |