Low, Spongy Pedal After Caliper Replacement
A 2015 medium-duty truck with four-wheel disc brakes, hydro-boost, and ABS comes in after another shop replaced both front calipers and hoses. The complaint is a low, spongy pedal. The pedal pumps up after three strokes but sinks slowly toward the floor when held with steady pressure for 30 seconds. Brake fluid level is correct and there are no external leaks. The shop used a pressure bleeder at 15 psi but did not perform the scan-tool ABS bleed procedure. Both calipers are mounted with the bleeders at the top, and the hoses are not twisted. The master cylinder was not replaced. The truck stops, but the driver says the pedal is lower than before.
1. Differentiate air in the system from internal master-cylinder bypass using the symptoms. What test would you perform?
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A pedal that pumps up and becomes firm indicates air in the system. A pedal that holds firm and then sinks slowly under steady pressure indicates a leak - external or internal. This truck does both: it pumps up, then sinks when held, so air is present and an internal master-cylinder cup bypass is also possible.
Perform a master-cylinder isolation test: block the outlet ports or clamp the flex hoses with approved tools, then hold the pedal. If it still sinks with the outlets blocked, the master cylinder is bypassing internally. Also check for fluid loss into the hydro-boost; a failed hydro-boost seal can draw fluid out of the master cylinder without an external leak. Verify pedal free play and pushrod adjustment before condemning the master cylinder.
2. Explain why the ABS hydraulic unit may still have air and what bleeding sequence you would use.
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The ABS hydraulic unit contains valves and a pump that can trap air. Pressure bleeding at the calipers may not move air out of those passages, especially after the system was opened at the front calipers.
Use the manufacturer's scan-tool ABS bleed procedure to cycle the valves and run the pump, then bleed each wheel in the specified order. Keep the reservoir full and never let the pump run dry. After the scan-tool procedure, pressure-bleed again and verify a firm pedal with the engine running. If the pedal remains soft, repeat the ABS bleed and check for air trapped at the master cylinder.
3. Six weeks later, the right front wheel is hot and drags. What are the likely causes, and how would you check them?
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Likely causes are a sticking caliper piston or slide, a collapsed or internally restricted hose, trapped residual pressure from the master cylinder or a valve, or a wheel bearing problem. Check by cracking the right front bleeder with the wheel dragging: if the wheel releases and fluid squirts out, pressure is being trapped upstream in the hose, ABS unit, or master cylinder. If it still drags, the caliper or mechanical components are at fault.
Inspect the hose for twist or internal collapse, check caliper slide pins and piston retraction, verify pedal free play, and check hub temperature and bearing play. Replace the caliper or hose only after the pressure-trapping source is identified.