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Old 02-06-2004, 01:09 AM #1
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Dragging brakes can be hazardous on slippery roads

Dragging brakes can be hazardous on slippery roads
by Jim Kerr

Driving your vehicle straight down the road would seem to be a simple task, yet the last few weeks of winter weather have provided ample examples of vehicles that have difficulty keeping on the beaten path. Broken plastic, dented fenders and bent guard-rails are all testimony to the treachery of winter roads. Some of this is probably due to driver error, but sometimes the damage can be caused by vehicle problems that drivers may not even be aware of. Your vehicle's brakes are one of the areas that can cause problems.

Step on the brake pedal and the vehicle slows. Release the brake pedal and the vehicle will roll easily. That's how brakes should work, but corrosion, wear and time can cause brake pull or drag. Combine a little brake drag with icy roads and you have a vehicle ready to slide out of control.

When a driver steps on the brake pedal, brake fluid is pushed from the brake master cylinder into the brake lines and down to the wheels. On disc brakes, the fluid flows into the calipers and pushes on a piston, which pushes the brake pads against the rotor. Drum brakes, still used on the rear of many vehicles, have the brake fluid flow into a wheel cylinder between two pistons, which force the brake shoes out against the inside of the brake drum. Each system can have problems that cause brake drag.

On disc brakes, the most common problem is corrosion on the caliper slides. Many vehicles have "floating" calipers. The caliper is like a big C clamp with a piston on one side. The piston pushes against a brake pad, which pushes against the rotor. The force of the piston against the rotor causes the caliper to move on its slides so the second brake pad on the other side of the caliper is also clamped against the rotor. The brake fluid pressure is high enough to force the caliper to move to the clamped position, but sticking slides prevent it from releasing completely, so the brake drags.

A caliper that is slightly sticking may not be noticeable when road surfaces and traction are good, but on icy roads it can cause a wheel to slide when the brakes are released and the vehicle pulls to one side. The fix? Remove the brake caliper and clean and lubricate the slides.

Multi-piston brake calipers, sometimes used on high performance cars, are fixed or bolted in place. Pistons on each side of the caliper force the brake pads out. There are no slides, but these calipers can still have problems. Just like the single piston caliper, the multi-piston caliper uses the seals on the piston to retract the piston slightly when the brakes are released.

The piston seal is a square cut seal ring inside the caliper bore. When brake fluid pushes the piston out, the seal ring distorts slightly as the piston slides through it. When the pedal is released, the seal ring returns to its relaxed state and pulls the piston back slightly as it does. Corrosion on the pistons, dirt or sludge in the caliper bore or hardened seal rings can all prevent the piston from releasing, and the brakes drag. The fix? Rebuild or replace the calipers.

Drum brakes use fluid pressure to force the brake shoes into contact with the brake drum but use springs to retract the shoes away from the drum. Springs can break, corrode or lose their tension, causing the brake shoes to drag. The fix? Replace brake hardware (springs, retainers, hold down pins etc. when brake shoes are replaced. The parts are cheap.

Oil leaking onto brake shoes can also cause them to drag. Gear oil from the rear axle on cars and trucks with solid rear axles or leaking brake fluid can contaminate the brake linings. Stains or wet spots on the bottom of the drum brake housings are an indication of leaks and the brakes should be inspected for contamination. Contaminated brake shoes cannot be cleaned so they must be replaced.

Many of the vehicles that I see slide out of control on icy roads start to slide when the brakes are released. The uneven braking caused by dragging brakes is enough to challenge even a skilled driver. Other items such as tire pressure, wheel alignment and steering angle also affect vehicle control but that must be left for another time. Ensuring your brake system is in top-notch condition is a great start to keeping it going straight on slippery roads.
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