A5 BRAKES // FLAGSHIP DIAGNOSTIC
2017 SUV
3.5L V6 · 75,000 mi · pedal slowly sinks under hold
Customer concern
"The brake pedal slowly sinks to the floor when I'm sitting at a stoplight. I have to pump it to hold the car. Started a couple weeks ago. No warning lights."
Pre-checks
DTC: None BRAKE FLUID: At MAX line, clean amber — OK PEDAL FEEL: Normal pedal, builds firm — OK PEDAL HOLDS?: Drops 1-2" over 30 sec under hold — FAIL EXTERNAL LEAKS: None at calipers/hoses/MC/cyls — OK PADS/ROTORS: Within spec, even wear — OK
Fluid is good. No external leaks. The pedal SINKS under hold — fluid is going somewhere internal.
Step 1 · Pedal feel test
Differentiate a spongy pedal from a sinking one. Apply steady, moderate pressure and hold — watch what the pedal does over 30 seconds with the engine running.
Pedal travel · cross-section
Hold · 0 secPedal travel from rest
A spongy pedal indicates air in the system. A pedal that builds firm then slowly sinks under sustained pressure indicates what?
Step 2 · External leak inspection
On the lift. Walk every hydraulic joint — calipers, flex hoses, hard lines, the master cylinder body, and the reservoir. If fluid were leaving the system, you'd find it here.
Under-vehicle inspection · findings
ALL 4 CALIPERS: Dry, no fluid residue — OK ALL 4 FLEX HOSES: No swelling, no wet spots — OK HARD LINES: Dry, no corrosion pinholes — OK MASTER CYL EXT: Dry at reservoir + booster pushrod— OK RESERVOIR LEVEL: At MAX (didn't drop) — OK
No fluid is leaving the system. So where is it going internally?
No external leaks + pedal sinks under hold + fluid level stays normal. What internal failure fits this exact pattern?
Step 3 · Bypass confirmation test
Engine off — no power boost in play. Pump the pedal until hard, then hold steady pressure for 60 seconds by hand and watch the pressure transducer.
Master cylinder · primary + secondary seals
Hold · 0 sec · engine OFFPressure transducer · line PSI
Pedal travel under hold
A pedal that holds for a second then slowly sinks — with pressure bleeding off — under sustained hold with the engine OFF confirms what?
Step 4 · Fluid & bore inspection
Confirmed bypass — now find the upstream cause. Test the fluid and look at the bore the seals ride against.
Brake fluid analysis
MOISTURE: 3.5% water content — FAIL SPEC: ≤ 3.0% water FLUID AGE: Last flush "never" — FAIL
Master cylinder bore
MC BORE: Surface corrosion / pitting — FAIL LINES: OK externally — OK ABS UNIT: Functional (at-risk) — WATCH
Old fluid with high moisture + a corroded master cylinder bore — how does this relate to the failure?
Step 5 · Root cause & repair
Internal bypass, corroded bore, water-saturated fluid. Tie it together — what's the real fix that keeps this customer from coming back?
What is the root cause and the full repair for this customer?
Diagnosis Closed.
Master cylinder replaced and the system flushed — no parts cannon, no return visit.
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A5 · BRAKES
Profile +14 pts
Real-shop equivalent
In a real bay, the parts cannon throws calipers, hoses, even an ABS module at a sinking pedal — none of which is the fault. Isolating an internal master cylinder bypass with an engine-off hold test saves ~1.5–2 hours of chasing phantom leaks, and catching the neglected fluid prevents the comeback. A tech who's 14% sharper on brake hydraulics recovers ~290 billable hours a year. Every scenario you train builds the cert and the daily diagnostic edge.