Hydraulic Brake Pedal Fade on a Cutaway Bus
A cutaway transit bus with a hydro-boost hydraulic brake system has a low, spongy pedal that fades toward the floor when held at a stoplight. The reservoir is at the full mark and the fluid is dark. There are no external leaks. The bus was recently serviced for a power steering hose replacement. The hydro-boost unit was not replaced. The pedal is firm when the engine is off and pumps up, but drops when the engine is running. The rear wheel cylinders were replaced 6 months ago.
1. What does the change in pedal feel between engine off and engine running tell you about the fault?
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A pedal that is firm with the engine off and drops with the engine running points to the hydro-boost or its hydraulic supply, not a conventional air-in-lines problem.
Reasoning:
- Engine off = no boost; the master cylinder and hydraulic circuit are isolated from boost pressure. A firm pedal here means the master cylinder and wheel circuits can build pressure.
- Engine running = boost applied. A pedal that sinks indicates boost pressure is being applied to the master cylinder without a corresponding pedal input, or fluid is bypassing internally in the hydro-boost.
Most likely: internal leak in the hydro-boost unit, often caused by contamination or a power steering hose replacement that introduced debris. Check the hydro-boost for internal bypass and verify the power steering pump pressure and return flow. Also confirm the hose was the correct type and not restricted.
2. The hydro-boost is suspected. How would you confirm an internal hydro-boost fault versus a master cylinder fault, and what is the repair?
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Isolate with two tests:
- Hydro-boost bypass test - with the engine running, hold steady pedal pressure. If the pedal slowly sinks and the power steering fluid level rises or the return line shows excessive flow, the hydro-boost is bypassing internally.
- Master cylinder isolation - cap the master cylinder ports or use a plug kit and press the pedal. If the pedal holds firm with the boost isolated, the master cylinder is good and the hydro-boost is at fault.
Also check:
- Power steering pump pressure and flow against spec.
- Fluid condition - dark fluid suggests contamination; flush the system.
Repair: replace the hydro-boost unit, flush the power steering/hydro-boost circuit, replace the filter if fitted, and bleed per OEM procedure. Confirm pedal height and firmness with the engine running before road test.