Lesson 1/Learning Event 3
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.
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.
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