A Pedal That Keeps Sinking After a Fuel-Station Bleed
A light commercial vehicle with disc brakes at the front and drum brakes at the rear is reported with 'the pedal needs pumping, and it pulls to the right when I brake hard'. The driver says the brakes were bled at a fuel station three weeks ago and he was told it was 'only air in the line'. Your inspection: with the engine running and steady heavy pressure held on the pedal, the pedal sinks about 25 mm in 30 seconds; the reservoir is at MIN and the cap seal is wet; the left rear drum is wet with dark fluid around the wheel cylinder dust boot while the right rear is dry; front pads are 8 mm; the fluid is dark brown; the vehicle has done 96,000 km and there is no record of a fluid change. No fluid puddle is visible under the parked vehicle. The right front caliper slides freely on its pins.
1. Explain why 'only air' does not fit these findings, and name the most probable fault. What tests would confirm it?
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Why air alone does not fit: air in a hydraulic circuit makes the pedal spongy and low, but once pumped it should hold position under steady force. A pedal that keeps creeping down 25 mm over 30 seconds while the force is held constant means fluid is moving out of the pressurised circuit somewhere - either past the master cylinder seals back into the reservoir, or out of the system altogether.
The second clue is the reservoir. It is at MIN with a wet cap seal, and the left rear wheel cylinder boot is wet while the right is dry. Fluid is being lost externally, slowly enough that it burns off or sits inside the drum rather than forming a puddle. Air is therefore a consequence of the leak, not the cause.
Most probable fault: a leaking left rear wheel cylinder, and possibly a master cylinder with internal bypass as a secondary fault.
Confirming tests:
- Clean the wheel cylinder and drum area dry, have an assistant hold firm pedal pressure for a minute, and watch for fresh fluid at the boot.
- Peel back the dust boot: fluid inside the boot means the cup seal has failed, regardless of mileage.
- Check the master cylinder by blanking its outlet ports or clamping the flexible hoses and repeating the pedal-hold test; if the pedal still sinks with the rest of the circuit isolated, the master cylinder is bypassing internally.
- Re-check fluid level after the test and inspect the rear brake linings for contamination.
2. Why does the vehicle pull to the right, and what does that tell you about the left rear brake?
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Reasoning: a vehicle yaws towards the side producing the greater braking force. If the left rear brake were working normally and the right rear were weak, the vehicle would pull left. It pulls right, so the left rear brake is the weaker one - which is exactly where the leak is.
Why the leak weakens that brake: fluid loss and the air it admits reduce hydraulic pressure at that wheel cylinder. At the same time, the leaking fluid contaminates the lining and drum surface. A lining soaked with brake fluid loses friction, so even when pressure reaches the cylinder, that brake does less work. The result is both a longer pedal and a right-hand pull.
What must be ruled out before blaming the rear: a dragging or seized right front caliper can also pull a vehicle right. The check already done - the right front caliper slides freely on its pins - makes a front-side drag unlikely, but the front brakes should still be inspected for even pad contact and free pistons after the rear is repaired.
Also note: rear brakes contribute a smaller share of total braking on a light commercial vehicle, so a rear-only fault may give only a slight pull at moderate pedal effort and a much clearer pull at hard braking. That matches the driver's report of pulling only when braking hard. Direction of pull is diagnostic evidence, not a coincidence.
3. What must be done before the vehicle goes back on the road, and what would you tell the driver?
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Work before release:
- Renew the leaking wheel cylinder; on a commercial vehicle, renew or at least strip and inspect the opposite side as well, so braking stays balanced.
- Renew the contaminated linings on that side (and the opposite side as a pair) - cleaning fluid-soaked friction material is not acceptable. Clean the drum with brake cleaner and check for scoring, ovality and a seized adjuster or handbrake link.
- Find out why the cylinder failed: a torn dust boot, corroded bore from old fluid, or a seized piston. Fit a quality cylinder, not the cheapest.
- Renew the brake fluid completely. It is dark brown, has no change record, and DOT-type fluid absorbs moisture, which lowers the boiling point and corrodes the bores. Pressure-bleed in the correct sequence, keep the reservoir topped up, and do not let it run dry.
- Verify function: pedal holds firm at constant force with no sink over 30 seconds, no seepage at any joint or boot, correct fluid level, handbrake adjustment, and a road test at 40 km/h in a safe area with straight-line braking and no pull.
To the driver: the fuel-station bleed only removed the symptom and the leak was still there; a sinking pedal means a leak, and it should be checked immediately rather than pumped. Advise a fluid change at the manufacturer's interval (typically two years or 40,000 km, whichever comes first), and ask him to return if the pedal feel changes or the level drops. Note all findings and parts replaced on the job card.