Soft Brake Pedal or Sinking Pedal: What It Means and Why You Should Not Drive It

About this video

WHAT TO DO IF IT HAPPENS WHILE DRIVING:
Pump the pedal rapidly to try to build pressure
Downshift to let the engine slow the vehicle
Use the parking brake gradually, not all at once
Steer to a safe shoulder and get off the road
Check the brake fluid reservoir once stopped
Do not keep driving it. Have it towed

HOW THE SYMPTOM POINTS TO THE CAUSE:
Pedal sinks slowly under steady pressure with no visible leak, that is almost always a failing master cylinder bypassing internally
Pedal is soft and spongy but holds, that is usually air in the lines or old contaminated fluid
Pedal goes to the floor and fluid is low, you have an external leak at a line, hose, caliper or wheel cylinder
Pedal is hard and the car barely stops, that is a brake booster or vacuum supply problem
Pedal pulses or is low only after hard braking, look at ABS operation and rotor condition

COMMON CAUSES OF A SINKING BRAKE PEDAL:
Failing master cylinder with internal seal bypass
Air trapped in the brake lines
Brake fluid leak at a rubber hose, hard line, caliper or wheel cylinder
Leaking rear wheel cylinder on drum brake vehicles
Old moisture contaminated brake fluid boiling under heat
Corroded and swelling brake line
Failing brake booster or leaking vacuum check valve
ABS module or ABS pump failure
Excessive caliper piston travel from worn out pads
Rear brake shoes out of adjustment
Improper bleeding after a brake job