Intermittent Brake Pressure Loss - Failing Master Cylinder Internal Seals

Brakesintermediate✓ OP Confirmed

Problem

Brake pedal suddenly sinks halfway to the floor while driving, as if a brake line had blown. Only minimal stopping power remains from the second brake circuit. The issue is intermittent - brakes can feel perfectly firm again after sitting, then fail again unexpectedly.

Brake pedal sinks halfway to floor during normal drivingDramatic loss of braking powerIntermittent failure - brakes return to feeling 100% firm on their ownNo visible fluid loss or leaks anywhereBrake fluid level unchangedNo pulling to either side

Root Cause

The internal seals inside the original master cylinder were aged and degraded, allowing brake fluid to internally bypass rather than build pressure in the brake lines. The seal deformation was intermittent - sometimes the seal would seat properly and brakes would feel normal, other times it would fold over and lose pressure.

Confidence

High confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted November 2020 on SupraForums.com