Soft Brake Pedal / Pedal to Floor - Air Trapped in ABS Module

Brakesintermediate✓ OP Confirmed

Problem

Brake pedal was soft and would slowly sink almost to the floor with little braking force. Pumping the pedal several times would build some pressure and brakes would work better temporarily. Problem persisted through replacing pads, rotors, rubber lines (with stainless steel), all calipers, brake booster, booster hoses, booster check valve, and repeated bleedings. Normal bleeding produced no air bubbles and fluid was difficult to flow through the system.

Brake pedal slowly sinks almost to the floorVery little braking force on first pressPumping pedal 3-4 times builds pressure and brakes work wellWheezing sound under dash when brakingRPMs drop ~400 and engine runs rough when pressing brake pedalNormal brake bleeding produces no air bubblesFluid very difficult to flow during normal bleeding

Root Cause

Air was trapped inside the ABS hydraulic module. Because the ABS lines exit from the top of the unit, air bubbles become trapped inside and cannot be purged through conventional brake bleeding at the calipers. The trapped air compresses when the pedal is pressed, causing the soft/sinking pedal feel.

Confidence

High confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted November 2021 on SupraForums.com