Hydraulic Pedal Fade After a Rear Drum Service
A Type A school bus with a hydraulic split-diagonal system and rear drum brakes had rear shoes and wheel cylinders replaced. The technician bench-bled the master cylinder, gravity-bled the rear circuit, and test drove. The pedal is firm at rest but sinks slowly when held with light pressure at a stoplight; the driver also reports the pedal goes lower after several hard stops in a row. No external leaks are visible at the wheels or lines. The master cylinder reservoir is at the full mark and the fluid is dark. Rear brake adjustment was set with a slight drag. The bus has 62,000 miles and the master cylinder is original.
1. Distinguish between the two pedal symptoms (slow sink at a steady hold vs low pedal after repeated hard stops) and explain the likely cause of each.
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These are different failure modes and should be diagnosed separately:
Slow sink while holding light pressure is the classic sign of an internal bypass leak in the master cylinder (worn primary cup/seal) or a leaking proportioning/combination valve. Fluid is escaping past the seal internally, so the pedal creeps down with no external leak. Dark fluid and an original master at 62,000 miles support internal wear. A residual-air pocket can also cause a soft/spongy pedal, but air usually gives a spongy feel, not a steady creep.
Pedal goes lower after repeated hard stops points to fade or knock-back: either brake fluid boiling (moisture-contaminated fluid, dark fluid is a clue) or shoe knock-back from poor adjustment/hardware. Repeated hard stops heat the drums; if the fluid has absorbed moisture, it can boil and create gas, giving a low pedal that recovers after cooling.
So: the steady sink is a hydraulic sealing problem (master cylinder), and the repeated-stop low pedal is a heat/fluid or adjustment problem. Both need correction before release.
2. Given the findings, list the repair steps in order and state how you would verify the fix, including what a proper pedal-hold and fluid test should show.
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Order of repair:
- Inspect and replace the master cylinder (bench-bleed the new/rebuilt unit). Do not rebuild if the bore is scored; dark fluid and mileage favor replacement.
- Flush the entire system with fresh DOT 3/4 fluid until clear, including the ABS modulator if equipped, following the manufacturer's bleed sequence (often RR, LR, RF, LF or per spec).
- Re-verify rear adjustment and hardware; confirm shoe-to-drum clearance and that the self-adjusters work.
- Check the combination/proportioning valve for external seepage and correct operation.
- Inspect for contamination source (moisture, wrong fluid) and note it.
Verification:
- Pedal hold: with the engine running (if power assist), hold firm pressure for at least one minute; the pedal must not creep down. A slow sink after repair means a remaining internal leak.
- Repeated hard stops: perform several hard stops from moderate speed in a safe area; pedal height and feel must remain consistent, not progressively lower.
- Fluid check: fluid should be clear and at the full mark; check for aeration.
- Road test and confirm no warning lamp and no pull.
Document before/after pedal behavior and fluid condition.