MILFORD, MI – General Motors Corp. says its ’05 fullsize pickups will be equipped with an all-new braking system that improves performance and is more pleasing to use.

The rub for GM engineers: Their one-step-back-two-steps-forward solution requires a bit of explanation. At first blush, the uninitiated might be convinced GM actually is cutting back on the pickups’ braking ability.

New front disc is larger, twin-piston caliper stiffer.

When the current Chevrolet Silverado and GMC Sierra pickups (internal designation: GMT800) were launched as ’99 models, GM bragged they were the industry’s first fullsize light-duty pickup trucks to come standard with 4-wheel disc brakes.

Until then, pickups – whose rear wheels typically don’t contribute much to the act of slowing the truck – traditionally used less-powerful, less-costly drum brakes for the rear wheels.

The new braking system for the ’05 Silverado and Sierra returns to drum brakes for the rear wheels, but it is not a retrograde move.

GM engineers, along with co-developer Robert Bosch GmbH, are leveraging their knowledge that pickups rarely use their rear brakes as hard as the fronts. They have taken the savings gleaned from using rear drum brakes and spent it on a serious upgrade of the trucks’ front brakes and other braking-system components.

Because the ’05 trucks’ harder-working front brakes now are considerably more powerful, GM and Bosch promise the new system delivers meaningful performance improvements, despite the return to drums at the rear.

Equally important, the new system, engineers say, will provide drivers with better feedback, require less pedal effort and deliver generally better overall response and “feel.”

Jully Burau, assistant chief engineer-fullsize trucks, says GM’s truck engineers pursued the new braking-system design because of continuous-improvement initiatives. “It’s an evolving product. We’re always driving for better quality, reliability and durability,” Burau says.

New master cylinder stiffer, enhances pedal “feel.”

She also says GM targeted the braking system because of J.D. Power and Associates initial-quality ratings and other consumer-clinic data that show customers believe there is plenty of room for improvement in the performance and feedback of pickup brakes – and because GM was not pleased with the Silverado/Sierra’s rating vs. rivals from Ford Motor Co., DaimlerChrysler Corp. and Toyota Motor Corp.

Issues most often cited include high brake-pedal effort, “mushy” feel from the pedal and the feeling the brakes don’t provide enough stopping power.

Fixing those problems, says Ken Hamann, director-brakes and chassis control systems, led GM and Bosch to a host of solutions that not only deliver quantifiable reductions in stopping distance and time but also generate more responsive feedback to the driver.

The primary hardware upgrade for the ’05 light-duty pickups is 1 in. (2.5 cm) larger-diameter front rotors and twin-piston calipers that are significantly larger and stiffer than the current twin-piston squeezers. Hamann says the larger rotor provides a 55 sq.-in. (355 sq.-cm )increase in rotor swept area, a tremendous aid in generating more stopping force.

The upgraded rotor/caliper components also deliver an important ancillary benefit, says Hamann. Their better stopping power means engineers could eliminate a booster previously necessary to comply with a federal safety standard that dictates the amount of allowable brake-pedal pressure in the event the braking system’s power-assist is inoperable.

The new brakes are powerful enough to eliminate the costly and space-consuming booster, which had to be added solely to provide extra pedal assist in the rare event of total power loss.

Hamann says the new brake system also includes a totally new master cylinder and booster that is stiffer and more efficient. That stiffness, combined with the extra beefiness found in the new calipers, helps to reduce the spongy feeling about which customers complain. By taking compliance out of these components, Hamann says customers will “experience more confidence with the pedal.”

GM engineers, in fact, developed in the early 1990s a matrix to help quantify the sometimes esoteric subjective facets of brake-system feedback. Their Brake Feel Index (BFI) assigns a vehicle a single BFI score based on a composite of weighted variables that contribute to a customer’s overall impression of the braking experience.

GM thought it could – and should – improve its fullsize pickups’ BFI scores, says Burau. “We’re focused on the customer interface,” she says.

The new system, says Hamann, registers a 41% BFI improvement vs. the ’04 brake system. GM’s numerous other GMT800-derived vehicles, including its fullsize SUVs, continue to use the current disc/disc system.

Hamann says the new trucks’ improved braking system will reduce stopping distance from 60 mph (97 km/h) by 85 ft. (26 m) and stopping time by 2 seconds. Meanwhile, the driver will not have to exert as much effort on the brake pedal, and the pedal will feel more responsive and deliver more of the confidence Hamann says is important to drivers, particularly when the pickup is heavily loaded.

As for those “new” old drum brakes at the rear, they are no slouches; Hamann says the 11.6-in. (29.5-cm) diameter drums are the largest in a light-duty vehicle.

But he knows the perception of returning to a less-sophisticated component is undeniable. That’s why GM and Bosch engineers point to the improved performance and feel of the new system that takes advantage of pickups’ unique braking behavior.

And why, Hamann says, he hopes pickup customers and auto critics try the new system before judging.

“They need to feel, touch and understand what we went through to make that call (to return to drum brakes),” he insists.