Long Pedal After Caliper Replacement on an ABS Van
A 2017 light commercial van (2.0 diesel, front discs and rear drums, ABS and ESP) arrives after another workshop replaced both front calipers and hoses. That technician gravity-bled the system with DOT 4 and sent the van out. The driver reports the pedal now travels almost to the floor on first application, though it pumps up and feels firmer on the second stroke. The van stops, but the invoice says only "brakes bled, road tested". Fluid level is at MAX, there are no external leaks, and all wheels turn freely. The scan tool shows no ABS faults. The workshop is heavily booked, the customer needs the van for deliveries tomorrow, and the supervisor asks you to decide whether to re-bleed, replace the master cylinder, or investigate the rear brakes, and whether the vehicle can be released if the pedal still has some travel.
1. How would you distinguish trapped air in the ABS hydraulic unit from misadjusted rear drum brakes or a failing master cylinder, using pedal feel and basic workshop checks?
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Start with the pedal behaviour because it separates volume/clearance faults from a pressure leak. With the engine running, apply moderate pressure and hold it for 30–45 seconds. A pedal that continues to creep down indicates an internal bypass, usually the master cylinder; a pedal that is low at first but comes up and stays firm after two or three pumps points to air or excess clearance somewhere in the system. Check the rear drums next: lift the rear axle, adjust the shoes until they just contact the drum, then back off to specification. If pedal height improves, rear clearance was part of the problem. Trapped air in the ABS hydraulic unit is plausible after calipers and hoses were replaced, because gravity bleeding does not cycle the ABS valves and pump. Use the scan tool to run the manufacturer's ABS bleed routine while pressure bleeding, then re-bleed the corners. Do not clamp flexible hoses or push an old master cylinder pedal to the floor, since that can damage seals in the unused bore. If the pedal still creeps, isolate the master cylinder with the correct test fittings before condemning it. Replace parts on evidence, not guesswork.
2. Set out your bleed procedure and your release decision, including what you record and what you tell the customer.
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Use fresh, sealed DOT 4 and a pressure bleeder at 1.5–2.0 bar, following the manufacturer's sequence, typically right rear, left rear, right front, left front. Keep the reservoir above minimum and draw enough fluid at each corner to clear old fluid and air. Then connect the scan tool, run the ABS bleed/valve-cycling function as specified, and re-bleed the corners. With the engine running, check pedal height, free play and firmness, and confirm no fluid loss or warning lights. Release the van only if the pedal is consistently firm with no creep, the fluid level is correct, and a low-speed road test, including an ABS self-test on a safe surface, is satisfactory. If the pedal still has excess travel, keep the vehicle, document the measured pedal height before and after, the bleed volumes, the scan-tool report and the customer's authorisation, and arrange a loan vehicle or reschedule. Tell the customer plainly that the brakes are not yet right and that releasing the van would be unsafe. Avoid promising a completion time you cannot support; a second bleed after the ABS routine may be needed.