1994-1999 Pontiac Bonneville Brake Noise: Squeal, Grinding and Pulsation Causes

By Core Brake Parts Tech Team — Circleville, OH warehouse

On a 1994-1999 Pontiac Bonneville, a high squeal usually means worn front pads or vibrating pad hardware, a metal-on-metal grind means the friction material is gone, and a pulsing pedal almost always traces to thickness variation on the front rotors rather than a truly warped rotor. These cars carry front disc brakes and, on the trims we checked, rear drum brakes, so the noise can come from either end. Here is how to tell which, and what fixes it.

How the Bonneville brake system is laid out

The eighth-generation Bonneville is a front-drive, V6 sedan weighing roughly 3,400 to 3,550 pounds depending on trim. Published specifications for the 1997 Bonneville SSE list front disc brakes, rear drum brakes measuring 8.85 x 1.77 inches, and four-wheel anti-lock brakes, with a curb weight of 3,548 pounds. The 1999 Bonneville SE is listed the same way: front discs, rear drums at 8.9 x 1.77 inches, four-wheel ABS, and a curb weight of 3,446 pounds.

That matters for noise diagnosis. The front discs do most of the stopping on a heavy front-drive sedan, so they wear fastest and make most of the noise. The rear drums work less hard but bring their own sounds: scraping from a worn shoe, a rhythmic noise from a scored drum, or a clunk from loose hardware. Because options varied by trim and year, pull a rear wheel or look through the wheel spokes to confirm whether your car has a drum or a rotor at the back before ordering parts.

Squealing: the most common Bonneville brake noise

Wagner Brake puts it plainly: vibration is the main source of noise in a brake system. A squeal is a pad, shim or caliper component vibrating at a frequency you can hear. On a Bonneville of this age, the usual sources are:

• Worn pads. Wagner lists grinding and squealing as signs of brake wear, and a high squeal when the friction material is completely worn out. Many pads carry a small metal wear tab that contacts the rotor on purpose to warn you.
• Tired hardware. Anti-rattle clips, springs and pins lose their spring tempering from brake heat. Wagner recommends replacing them rather than reusing them, which is why a brake hardware kit belongs in every front pad job on a car this old.
• Missing or damaged shims. Bendix notes that shims form a barrier between the pad and caliper that stops small vibrations building into noise. A pad installed without its shim, or with the wrong treatment on the shim, is a classic squeal source.
• Seized or dry caliper slide pins. Bendix lists seized slide pins as a cause of uneven wear, drag and noise. A caliper that cannot float keeps one pad lightly dragging.
• Rust and grit. Rust on the hub face or on the caliper surfaces where the clips sit keeps pads from sitting parallel to the rotor.

A light squeal on the first few stops of a damp morning that goes away is surface rust on the rotors and is normal. A squeal that is there on every stop, or gets louder over a few weeks, is telling you to inspect the pads.

Grinding: stop driving on it

A harsh grinding or growling noise usually means the pad friction material is gone and the steel backing plate is cutting into the rotor. Wagner lists lack of periodic pad inspection as a path to pulling, rotor damage and excessive noise. Once the backing plate has touched the rotor, the rotor is almost always scored too deep to reuse, so the repair becomes pads and rotors instead of just pads.

Grinding can also come from the rear. On drum brakes, a worn-through shoe lining lets the rivets or shoe table scrape the drum. You will often hear it most at low speed as you come to a stop, and it may change when you set the parking brake. Other grinding sources worth ruling out before buying parts:

• A small stone or rust flake trapped between the rotor and the splash shield (a scraping noise that comes and goes).
• A rusted rotor edge lip that the pad rides against as it wears.
• Deep scoring on the rotor face. Bendix notes that scored rotors can be machined only if they stay within the minimum thickness cast or stamped on the rotor.

Pulsation: why the pedal pushes back

If the pedal pulses and the steering wheel shakes when stopping from highway speed, the cause is almost always the front rotors. Power Stop explains that the real culprit is disc thickness variation (DTV), the difference between the thickest and thinnest spots of the rotor as it turns. Power Stop states that when thickness varies by more than 0.006 inch (0.15 mm), replacement may be needed.

DTV develops from several causes that Power Stop lists:

• Uneven pad material deposits. Pad material transfers onto the rotor in an irregular pattern, often when pads are not bedded in. Power Stop notes that deposits show up as visible smearing on the pad friction surface.
• Lateral runout. The rotor stays the same thickness but wobbles side to side, so it brushes the pads once per turn and wears unevenly over time. Rust on the hub face under the rotor is a common cause.
• Installation errors. Uneven lug nut tightening distorts the rotor. Power Stop advises tightening lug nuts with a torque wrench to the manufacturer's specification.
• Worn suspension parts. Damaged or worn suspension bushings can oscillate under braking and feel like a pulsing pedal.

A pulsation felt through the seat or the pedal but not the steering wheel can point to the rear drums, since an out-of-round drum produces the same once-per-turn push. A feel of rapid clicking through the pedal only during a hard stop on a slippery surface is the ABS working, not a fault.

How to diagnose which end is making the noise

1. Note when it happens. Noise only when braking points to pads, shoes or rotors. Noise while driving that stops when you brake often points to a wear tab or a dragging caliper.
2. Use the parking brake test. On a slow, safe roll in an empty lot, apply only the parking brake. If the noise appears, it is coming from the rear drums, since the parking brake works through the rear shoes on drum-equipped cars.
3. Look at the front pads. On most Bonneville wheels you can see the outer pad through the spokes. Compare the lining to the steel backing. If the lining is close to the backing, replace the pads now.
4. Feel for heat after a drive. Carefully compare wheel temperatures by holding a hand near each wheel (not on it). One much hotter than the other suggests a sticking caliper or a rear shoe that is dragging.
5. Pull the wheels and inspect. Check the rotor face for scoring or blue spots, check pad wear side to side, and check the slide pins for free movement. At the back, remove the drum and look for worn linings, broken springs or brake fluid on the shoes, which means a leaking wheel cylinder.

The fix: which parts to replace, and what to do at the same time

For front squeal or grinding, the standard repair on a Bonneville of this age is new front brake pads and new front brake rotors together. Rotors on a 25-plus-year-old car have usually been worn, rusted or machined before, and fresh rotors remove thickness variation as a variable. A ceramic brake pad is a good choice for a daily-driven sedan when you want low dust and quiet operation.

Do these at the same time:

• Replace the clips and springs with a hardware kit, and clean and lubricate the slide pins with a brake-specific lubricant.
• Follow the pad maker's instructions for the shim. Wagner notes that some pad designs with built-in insulators must have no compound placed on the insulator, while pads without shims should get the noise compound supplied in the box.
• Wire-brush rust off the hub face so the new rotor sits flat, and wash the new rotors with soap and water to remove dust and grit, as Wagner recommends.
• Torque the lug nuts in a star pattern to the value in the factory service manual.
• Bed in the new pads. Wagner's procedure is 20 gradual slow-downs from 50 mph to 20 mph with light pedal pressure, allowing at least 30 seconds between each one.
• If a caliper was sticking, replace it rather than freeing it up. Our brake calipers collection covers this job.

For rear drum noise, replace the brake shoes with a drum hardware kit, and inspect the drums and wheel cylinders. A drum that is scored or out of round should be replaced, and any wheel cylinder showing wetness should be replaced before the fluid ruins the new shoes.

A hissing sound when you press the pedal, especially with a hard pedal, is not a pad or rotor noise at all. It usually points to a vacuum leak at the brake booster or its check valve. We carry a power brake booster assembly for the 1994-1999 Bonneville V6 if the booster is the problem.

How hard is the job?

A front pad and rotor job on a Bonneville is a moderate driveway repair for someone with jack stands, a torque wrench and basic hand tools. The calipers bolt on conventionally and the rotors slide off the hub once the caliper bracket is removed, though a rusted rotor may need persuasion. Rear drum work is more involved because of the springs and self-adjuster, and it is easiest to do one side at a time so the other side serves as a reference. If a wheel cylinder or caliper is replaced, the system must be bled, and on an ABS-equipped car that step deserves care. If you are unsure, have a shop do the hydraulic work. Browse all brake parts to build your parts list.

Frequently Asked Questions

Does the 1994-1999 Pontiac Bonneville have rear disc or rear drum brakes?

Published specs for the 1997 Bonneville SSE and the 1999 Bonneville SE both list front disc and rear drum brakes with four-wheel ABS. Because equipment varied by trim and year, look behind a rear wheel to confirm before ordering rear parts.

Why do my Bonneville brakes squeal after new pads?

The usual causes are reused clips and springs, a shim left out or treated incorrectly, dry slide pins, or pads that were not bedded in. Replace the hardware, follow the pad maker's shim instructions and run the bed-in procedure.

Are my rotors warped if the pedal pulses?

Usually not. Power Stop explains that pulsation comes from disc thickness variation, often caused by uneven pad deposits, runout or uneven lug nut torque. New rotors and a proper bed-in fix it.

Can I just replace the pads if the brakes are grinding?

Rarely. Grinding usually means the backing plate has contacted the rotor, which scores it. Plan on pads and rotors together.

How do I know if the noise is from the front or the rear?

Try a slow roll in an empty lot using only the parking brake. On drum-equipped cars that applies the rear shoes, so a noise that appears then is coming from the rear.

Sources

1. CarSpecs: 1997 Pontiac Bonneville SSE - front disc, rear drum 8.85 x 1.77 in., 4-wheel ABS, 3,548 lb curb weight
2. CarSpecs: 1999 Pontiac Bonneville SE - front disc, rear drum 8.9 x 1.77 in., 4-wheel ABS, 3,446 lb curb weight
3. Wagner Brake: Brake System Noise - vibration as noise source, hardware replacement, shim treatment, rotor cleaning, bed-in procedure
4. Wagner Brake: Signs of Worn Brake Pads - squeal and grinding as wear signs, pulling and pulsation
5. Power Stop: Why Do I Have a Pulsing Brake Pedal? - disc thickness variation, 0.006 in. threshold, deposits, runout, lug nut torque
6. Bendix: Diagnosing and Rectifying Brake Noise - shims, seized slide pins, scored rotors