🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Brake master cylinder cup seal failure
💰 Estimated Repair Cost
$50 – $200 USD
Parts and labour estimate
🔧 Tools Required
Brake line wrench, bleed equipment
📋 Overview
The brake master cylinder cup seals fail with age causing the pedal to sink slowly under pressure. Distinct from an air lock — the pedal feels normal initially then sinks to the floor if held.
🩺 Symptoms
- Brake pedal sinks under sustained pressure
- pedal returns fully but sinks again
- no external fluid leak
🔍 Possible Causes
- Failed master cylinder cup seals
- scored master cylinder bore
- contaminated fluid
🛠️ Diagnosis Steps
Confirm by holding firm pedal pressure — pedal should not sink. Sinking under constant pressure confirms master cylinder failure.
✅ Most Confirmed Fix
Rebuild master cylinder or replace with reconditioned unit.
73% of users confirm this fix worked (16 reports)
🔗 Other Mercedes-Benz W115 Faults
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