HOW TO DO A COMPLETE BRAKE REPAIR WORK (part 2)

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BRAKE ROTOR RESURFACING

Rotor resurfacing is not necessary when the pads are replaced provided the rotors are in reasonably good condition (within acceptable runout, parallelism and wear specs). Even so, there are valid reasons for having the rotors resurfaced.

If a rotor is close to the OEM minimum thickness specification or cannot be resurfaced without exceeding the spec, then the rotor is near the end of its useful service life and replacement is recommended. If the rotor is at or below the minimum thickness specification, it must be replaced.

Worn rotors are dangerous rotors for three important reasons:

  • Worn rotors have less mass and are therefore less able to absorb and dissipate heat. This can elevate brake temperatures, which may lead to fading under hard use. Pad wear will also be accelerated.
  • Worn rotors are weak rotors. If not replaced, they may wear down to the point where they cause brake failure.
  • Worn rotors increase the distance the caliper piston(s) have to travel when the brakes are applied. If the distance is too great, there’s a danger of fluid leakage or the piston sticking.
  • Another reason for resurfacing rotors is if lateral runout or wobble exceeds OEM specs. You can attempt to reindex the rotor, but the best solution is to resurface the rotor on the vehicle with an on-car lathe. The lathe will cut the rotor true to the caliper mount or hub (depending upon how the equipment is mounted).

Resurfacing may also be needed if parallelism between the rotor faces exceeds OEM specs (generally about .0005 in.). If the rotor is warped or has hard spots, it will have to be resurfaced or replaced.

Hard spots that develop from overheating or uneven torquing of lug nuts can create raised areas on the surface that often extend below the surface. The metallurgical changes in the rotor will often cause the hard spots to return after a few thousand miles, so rotor replacement might be the best repair recommendation.

Regardless of the type of rotor resurfacing equipment used, always remove the least amount of metal necessary to restore the rotor surface. This will maximize rotor life. After turning the rotor on a lathe, some technicians will buff both sides of the rotor with a Flex Tool or sand the rotors with #120 to #150 grit sandpaper with moderate to heavy force for 60 seconds on each side to create a smooth, nondirectional or crosshatch finish. This is done to minimize the risk of noise with semimetallic pads.

REPLACING BRAKE HARDWARE

A complete brake job may also include new hardware. When replacing brake shoes, pay close attention to the condition of the hardware — especially the all-important return springs. Over time, heat weakens the springs and reduces their ability to pull the shoes back from the drum when the brakes are released. The brakes may begin to drag causing shoe wear to accelerate and fuel economy to take a noticeable dip. As the springs continue to age, they get weaker, stretch and may eventually break. MAP says there’s no reason to replace the springs unless they are obviously stretched, heat discolored, broken or damaged. But because of the risks associated with reusing high mileage springs and mounting hardware, you should install new return springs and other brake hardware — especially if the vehicle has a lot of miles on it.

If a parking brake cable is badly corroded and needs replacing, replace both cables to minimize the risk of future problems.

And don’t forget the wheel bearings. The greasable variety in older rear-wheel drive vehicles need to be cleaned, inspected, repacked with fresh grease, reinstalled with new seals and adjusted to specs. The sealed variety should be inspected for excessive play or roughness. If either type are not in acceptable condition, they need to be replaced.

Be sure to lubricate the shoe support pads on the drum backing plates with brake lubricant (never ordinary chassis grease). Also, lubricate the parking brake mechanism.

BLEED THE BRAKES

A complete brake job also includes fresh brake fluid. Bleeding is necessary for two reasons:

  • To remove air bubbles that may have entered the system while repairs were being made because of a leak or a brake fluid level that got too low. The air must be removed because it is compressible and can prevent a full, firm pedal.
  • To remove moisture contamination.

Brake fluid absorbs moisture over time, which lowers its boiling point and contributes to internal corrosion.

Changing the fluid periodically (every two years or when the brakes are relined) for preventive maintenance rids the brake system of unwanted moisture, restores the fluid’s boiling temperature and prolongs the life of the hydraulic components by minimizing the potential for internal corrosion. This can be an especially important consideration on vehicles equipped with antilock brakes (ABS) because of the high replacement cost of the hydraulic modulator assembly.

FINISHING THE BRAKE JOB

When you’ve completed your brake repairs, pump the brake pedal several times to confirm the pedal is firm before moving or driving the vehicle. If this is not done, the pedal may go to the floor and the brakes may be unable to stop the vehicle!

Do a short test drive at slow speed to confirm the brakes are working properly. The pedal should feel firm, the brakes should apply without pulling or grabbing, and you should hear no noise.

30/30/30 BRAKE PAD BURNISHING PROCEDURE

Perform 30 stops from 30 miles per hour with a 30-second cooling interval between stops. These stops will be performed at a decelerating rate of 12 feet per second or less. This means that it should be a gentle easy stop.

The 30/30/30 Burnish Procedure beds the pads and shoes into the rotor and drums. It also deposits the necessary friction transfer to the rotors and drums for optimum brake performance.

Do NOT go out and slam on the brakes as hard as you can. This can glaze the pads resulting in brake noise and reduced stopping performance. Take it easy on the brakes the first couple hundred miles of driving.

If the pedal feels soft or spongy, there may be air trapped in the brake lines. You should bleed the brake lines again to remove the air. If the pedal has too much travel, the self-adjusters inside the drums may need to be readjusted to reduce the clearance between the shoes and drums.

If the brakes pull or grab, a caliper may be sticking or there may be oil, grease or brake fluid on the pads or rotors. Inspect and clean as needed.

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