By Charles C. Roberts, Jr., Ph.D., P.E.
Analyzing the Cause and Consequence
Many claims these days uggest that automotive brake failure directly contributed to an accident. For those losses where brake malfunction is a suspected cause, the claims
investigator is obliged to look into the existence of evidence of brake failure.
Figure 1 to the right offers a brief review of an automobile brake system. The
brake pedal presses against a vacuum assist brake booster that pushes on a master
hydraulic cylinder, resulting in increased
hydraulic pressure in brake lines. The
brake lines are connected to brake calipers or brake shoe wheel cylinders, causing increased friction on the brake drum
or rotor, resulting in vehicle braking.
Older vehicle master cylinders supply
hydraulic pressure to all brakes via one
brake line (arrow Figure 2). Unfortunately, if a leak occurs in a brake line, then
the whole system loses pressure, and fails.
Modern brakes have dual master cylinders
that operate two independent systems,
which have proven more reliable. Modern
brake systems are also equipped with anti-lock features that have improved performance on certain road surfaces.
Figure 3 is a case study where the
insured claimed that the brakes failed,
causing an accident. The vehicle was an
older pickup truck with a single brake
system master cylinder. In Figure 3, corrosion has thinned the wall of the brake
tube, leading to brake tube failure, and
brake fluid leakage, ultimately causing
brake failure. This is a maintenance-relat-ed malfunction. In a dual master cylinder
system, this tube failure would result in
an approximately 50-percent loss of braking capacity, while hard braking capacity
would be lost altogether.
Figures 4A and 4B to the right illustrate
a failure where the entire master cylinder
broke loose from the engine vacuum as-
sist brake booster. The insured claimed
brake failure, which resulted in relatively
minor impact damage to the vehicle, as
shown in Figure 4A. Examination of the
brake system revealed that the master cyl-
inder had parted from the brake booster in
a manner such that when the brake pedal
was pressed, no actual braking occurred.
The damage to the vehicle did not support
the conclusion that the impact caused the
failure of the master cylinder or the brake
booster connection. Further examination
of the brake system revealed a tool mark
on the master cylinder housing shown in
Figure 4B. Apparently, during a recent