Spongy Pedal and Right Pull After a Roadside Brake Fluid Change
A car with front disc and rear drum brakes has covered 62,000 km. After a brake fluid change at a roadside garage, the driver reports a pedal that feels spongy and sinks slowly when held at a traffic light. The car pulls to the right during a 40 km/h stop, and the left rear wheel is cooler than the right rear wheel after the test. The fluid level in the reservoir has dropped about 10 mm in a week, but no puddle is seen under the car. The left rear drum is removed: the wheel cylinder is wet under its dust boot, and the brake shoe lining on that side is contaminated. The front pads have 6 mm lining; the discs show light scoring. The brake fluid is DOT 3 and was last changed four years ago.
1. Identify the two brake faults and explain how each contributes to the symptoms.
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Fault 1 is a leaking left rear wheel cylinder: the wet dust boot and contaminated shoe show hydraulic fluid is escaping. That reduces pressure at the left rear brake, so the right rear brake does more work; the car pulls to the right and the left drum stays cooler. The fluid level drops because fluid is being lost, and air can enter through the leak, making the pedal spongy. Fault 2 is old, moisture-contaminated DOT 3 fluid: it was last changed four years ago and DOT 3 absorbs water. Water lowers the boiling point and corrodes the system; it also contributes to a soft pedal. The slow sink when held is consistent with the external leak, but a master-cylinder internal leak should be ruled out after the wheel cylinder is repaired.
2. Explain why simply bleeding the brakes and topping up the fluid would be unsafe.
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Bleeding removes air but does not stop the leak. The wheel cylinder will continue to lose fluid and draw air back in, so the pedal may improve briefly and then fail again. The contaminated shoe lining cannot be cleaned reliably; brake fluid softens and glues the friction material, reducing braking on that wheel and causing a pull. A leaking wheel cylinder can suddenly fail under heavy braking, and brake fluid on hot drums or shoes is a fire risk. Topping up also hides the rate of loss. The correct approach is to repair the leak, replace contaminated shoes as an axle set, clean the drum, flush the old fluid, and then bleed.
3. Describe the correct repair sequence and the final checks before the car is returned.
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First support the car securely and remove the rear drums. Replace or overhaul the left wheel cylinder and inspect the right one; replace both if one is corroded. Replace the contaminated brake shoes as an axle set and clean the drum with brake cleaner; replace the drum if it is scored or beyond its diameter limit. Check the front pads and discs, and replace the fluid with the specified DOT 3 or DOT 4 fluid by flushing until clean at each bleed nipple. Bleed in the manufacturer's order, usually right rear, left rear, right front, left front. Check for leaks at all joints, a firm pedal that does not sink, correct fluid level, and free wheel rotation. Road-test at low speed, confirm no pull and no drag, and adjust the parking brake.