Pedal Sinks at a Stoplight Two Months After Rear Calipers

It is hydraulic. The booster only multiplies pedal force; it cannot create or absorb fluid, so it cannot make a pedal sink that otherwise holds.

Read the two tests carefully:

• Engine off, booster depleted, five pumps: the pedal is hard because there is no assist and relatively little force reaches the master cylinder. A master with a marginal cup seal can still hold at that low pressure. Firm here proves nothing except that there is no gross external leak.
• Engine running, 200 N hold: the booster multiplies that 200 N into well over 600 N at the master. The higher pressure is what exposes a cup seal bypassing fluid past its bore - or a downstream leak path that only opens under higher pressure.

The 30 mm of travel in 45 s at steady pressure is a slow sink, not a fast one. Slow = internal bypass or a small leak. Fast = a burst line or a failed seal at a caliper, which you have already ruled out visually and by the stable fluid level.

So: hydraulic, low-pressure-side-tested, and now you must separate the master from everything downstream of it.

Isolation by plugging.

1. Disconnect the brake lines at the master cylinder outlet ports and install the correct threaded plugs or a proper plug kit - never cap with a bolt wrapped in tape, and never let the master run dry of clean fluid.
2. Apply and hold firm pedal pressure with the engine running.
3. Pedal still sinks → the master is bypassing internally (worn bore, swollen or nicked cups, scored piston). Pedal holds → the master is good and the leak is downstream.

While the lines are off, inspect:

• Fluid colour and smell. Dark amber fluid can simply be age, but swollen, softened cup seals point to mineral-oil or wrong-spec fluid contamination, which is a system-wide problem, not a master-only problem.
• The ABS hydraulic unit for weepage at the pump body and around the motor, and the line fittings for fretting.
• Caliper piston boots - peel them back. A piston seal can leak into the dust boot with no external drip and no level drop, which is exactly this symptom on a recently replaced caliper.
• Flexible hoses for bulging under pressure (have an assistant hold pressure while you watch).

If the master passes, repeat the plug test at the ABS unit inlet to split the HCU from the wheel circuits. Any repair ends with a pressure-hold verification and a full flush, not a road test with a sinking pedal.

Rule out before condemning the master:

• Contamination. Swollen cups two months after a flush is a classic signature of the wrong fluid or a contaminated bleed bottle. If the cups are soft, the caliper seals are exposed to the same fluid and a master-only repair will fail again.
• Air still in the system. Air usually gives a spongy pedal rather than a slow sink, but a small pocket in the HCU can mimic it. Confirm with a proper scan-tool-actuated ABS bleed, not pedal pumping alone.
• Improperly seated or damaged caliper seals during installation, or a piston cocked in the bore.
• Bench-bleeding omitted on the replacement calipers, leaving air trapped.

Risk of guessing: replacing the master on a contaminated system puts new rubber in contact with the same bad fluid. You will be doing the job again, and the second time the customer will not be paying quietly.

Correct path: identify the leaking component with the plug test, then inspect fluid condition. If contamination is confirmed, quote a complete flush plus all rubber components exposed to the fluid - master, calipers, hoses - and state on the RO why the master alone is not a complete repair. Never use petroleum-based cleaners anywhere near the hydraulic system.