Originally Posted by MotorTrend
Road test editor Scott Mortara headed down the quarter mile in the bright-yellow Boss 302 Mustang, steadying his speed for Motor Trend’s standard brake test, just as he’s done a thousand times before. Back in the day, before anti-lock brakes and stability control systems and decent tires, you never knew quite which direction the car might be facing when you finished a brake test. These days, though, it’s fairly simple: At about 70 mph, you grenade the brake pedal and hang on. The electronic nannies keep the cars nice and straight, and our sophisticated on-board data-logging equipment automatically captures the distance it takes to decelerate from 60 mph to a standstill.
So when Scott punched the Mustang’s brake pedal and it went straight to the floor, it got his attention. He quickly ran the six-speed manual back through the gears, slowing the car enough to make it through the gap in the wall at the end of the track.
Total brake failure during regular performance testing is a very rare occurrence. I’ve had plenty of cars get squishy pedals and smelly pads after hot laps around a road course. But in more than 25 years of testing, I’ve never known of such a complete—and potentially catastrophic—loss of braking.
Earlier, this particular Boss 302 had repeatedly lapped Laguna Seca and been driven enthusiastically along some of California’s more challenging back roads. Scott had then driven it directly back to Motor Trend HQ in El Segundo so we could get the photos, video, and test numbers we needed for last month’s print and iPad editions, as well as our website and YouTube channel. Had the Mustang’s brake pedal gone to the floor any time in the week before it did, the consequences could have been horrifying.
So what went wrong? Scott heard a metallic snap as he braked. A quick look in the Mustang’s footwell revealed the pin connecting the pedal to the master cylinder clevis rod end had sheared.
The Mustang’s brake pedal arm is normally located between the two sides of the clevis, and even if the clevis pin should fail, after a very short interval the back side of the pedal arm would act against the base of the clevis with enough force to actuate the master cylinder and apply the brakes. But for some reason, the pedal arm in this car had been pinned to the outside of the clevis, on the right-hand side. This concentrated all the force acting on the clevis pin on a single point, causing it to shear; and when the pin failed, the pedal slid past the rod that actuated the master cylinder. The result? No brakes.
Ford reacted instantly to news of the failure. Our Boss 302 was collected and sent away for a tear-down and inspection. The Mustang line was shut down, and every brake assembly checked. Ford also reviewed its process to ensure incorrect attachment of the brake pedal arm to the master cylinder clevis rod end could not occur. We’re told no anomalies were found. And we haven’t seen anything awry on the Mustangs we’ve checked since.
How the arm was misassembled on our Boss 302 tester remains a mystery to Ford, and to us. The only thing we can think of is this particular car’s master cylinder had been removed after it left the factory, and the incorrect connection to the brake pedal arm made when it was reinstalled, although we have no proof either way. Based on the evidence, we’re prepared to chalk this incident up to a freakish combination of circumstances.
Modern cars are amazingly robust and reliable. However, stuff can—and occasionally does—go wrong. That’s why we test.