Spongy Pedal After a Rear Caliper
A 2018 sedan with ABS comes in at 16:00 with a seized rear left caliper. The shop replaces the caliper, reuses the fluid from a DOT 4 bottle opened seven months ago, and bleeds only the rear left corner. The reservoir ran dry while the caliper was off. The apprentice bled by pushing the pedal all the way to the floor, repeatedly. On collection the pedal is low and spongy: it firms up when pumped but sinks slowly toward the floor when held with steady pressure at a stop. Rear pads measure 7 mm, fronts 9 mm, no external leaks at any wheel or line, and the master cylinder was not touched. The customer is waiting and has a 40 km drive home after dark.
1. The pedal both pumps up and sinks slowly. Explain what each of those two symptoms tells you, and design the tests that separate air in the system from an internal master cylinder fault.
Read the worked answer
These are two different faults showing at once, and they must be separated before anything is signed off.
Pumping up to a firm pedal means the system has compressible material in it - almost certainly air, which is expected here: the reservoir ran dry so air entered the ABS hydraulic unit, and only one corner was bled.
Sinking slowly under steady pressure means fluid is escaping the pressurised circuit without an external leak. With no visible leak at the wheels, lines or caliper, the usual cause is an internal bypass in the master cylinder - fluid slipping past the primary cup back into the reservoir. A pedal pushed repeatedly to the floor is also the classic way to damage an older master cylinder, because the cup travels into a corroded, unworn part of the bore.
Tests:
- Pump the pedal and hold it firmly for 30–60 seconds with the engine off. Firm and steady = air only. Steady creep toward the floor = internal bypass or a leak.
- Clamp each flexible hose in turn and repeat the hold. If the sink stops when one corner is isolated, the fault is at that corner, not the master.
- Check the brake fluid level over the hold - a falling reservoir with no external leak points to the master or a leaking booster seal.
- Inspect under the dash at the pushrod and the booster for fluid.
Do not release the car on the pump-up alone.
2. State the correct bleeding procedure for this vehicle, including what to do about the ABS unit and the opened bottle of fluid.
Read the worked answer
Because the reservoir ran dry, air is inside the ABS hydraulic unit, and corner bleeding alone will not remove it.
- Fluid. Discard the seven-month-old bottle. DOT 4 is hygroscopic; an opened bottle absorbs moisture from the air and its boiling point falls, which is exactly what you do not want in a braking system. Use a sealed, in-date container, and never reuse fluid bled out.
- Master cylinder. Bench bleed or bleed at the outlet ports first so you are not pushing air through the whole car.
- Sequence. Follow the manufacturer's order - typically the corner furthest from the master first (rear right, rear left, front right, front left), and check the manual for any ABS-specific sequence.
- ABS unit. Use the scan tool's bleed or dump-valve routine to cycle the pump and solenoids so trapped air is pushed to the calipers, then bleed the corners again. On some vehicles this requires the scan tool before, not after, the manual bleed.
- Method. Two-person bleeding with a clear hose into a jar, watching for bubble-free fluid, or a pressure bleeder. Never push the pedal to the floor during bleeding - use short strokes to about half travel, or the master cylinder cup can be damaged in the unworn bore. Top up the reservoir after every corner so it never runs dry again.
- Final check. Firm pedal that holds under steady pressure with the engine running, correct fluid level, no leaks, then a low-speed road test including an ABS stop in a safe area.