Spongy Brake Pedal on a Yaoundé Taxi
You are a CAP auto mechanics trainee in a taxi garage in Yaoundé. A Toyota Corolla taxi is brought in with a spongy brake pedal. The driver says the pedal slowly sinks to the floor when he holds it at a red light, and the brake fluid level has dropped from MAX to MIN in two weeks. The fluid is dark. There is no visible leak at the wheels, but the rubber boot on the brake master cylinder is wet. A rear wheel cylinder was replaced three months ago. The vehicle is parked on a sloped concrete apron. You have a floor jack, two axle stands (one missing its locking pin), a pressure bleeder, DOT 3 fluid, and a set of spanners. The driver wants to leave the car and return in two hours because he has a school run.
1. Diagnose the most likely cause of the sinking pedal and fluid loss. Explain why simply topping up the fluid is dangerous.
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A pedal that sinks slowly under constant pressure, together with fluid loss and a wet boot on the master cylinder, points to a failed master cylinder seal. Fluid is leaking past the primary or secondary cup into the boot or the servo area, so pressure cannot be held. The dark fluid and the recent rear wheel cylinder repair mean I should also check that repair and the rear brake lines, but the wet boot is the strongest clue. Topping up the fluid would only hide the leak temporarily. The fluid has to go somewhere, and if it reaches the servo it can damage the diaphragm; more importantly, the brake can fail completely under hard braking. I would not road-test the taxi or let the driver leave with it. I would inspect the master cylinder, bench-bleed or replace it as required, check all lines and wheel cylinders, and only release the car after a pressure test and a safe road test.
2. The car is on a sloped apron and one axle stand is missing its locking pin. List the safety steps you would take before working underneath.
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I would not lift the car on the slope with a defective stand. First, I would ask the driver to move the car to level ground, or I would move it at walking pace with a helper. I would apply the handbrake, put the gearbox in first or reverse, and chock at least two wheels. I would use the floor jack only to raise the car, then place two sound axle stands under the manufacturer's jacking points. The stand missing its locking pin must not be used; I would replace it or use another rated stand. I would lower the car onto the stands, check that it is stable, and give it a firm shake before going underneath. I would never work under a car supported only by a jack, and I would keep the jack as a secondary support if it does not obstruct the work. Only then would I start the brake repair.
3. State the brake bleeding sequence for this left-hand-drive car and explain why DOT 5 silicone fluid must not be used.
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For a left-hand-drive Corolla, the usual sequence is right rear, left rear, right front, then left front - farthest from the master cylinder first, nearest last. I would keep the reservoir topped with DOT 3 fluid during bleeding, use the pressure bleeder at the manufacturer's recommended pressure, and check for clean fluid with no air bubbles at each bleed screw. DOT 5 is a silicone-based fluid and is not compatible with the glycol-based DOT 3 or DOT 4 system. Mixing them can cause seal swelling, poor pedal feel, and contamination; silicone fluid also does not absorb water in the same way and can affect ABS operation. I would use only the specified DOT 3 fluid, cap it tightly, and label the reservoir. After bleeding, I would check the pedal height and feel, inspect for leaks, and test the brakes at low speed before returning the taxi.