Brake pedal sinking when engine on

[xavierx]

2010 XC70 D5

Bled the brakes all the way around twice. First time used a vacuum bleeder and the second time did it Ye Old fashioned way

When engine is off, brake pedal feels solid

When you start the engine and press the brake, the pedal bites normally as expected, then sinks.

My first thought was a bad internal seal on the master cylinder.

However, I went into VIDA and pulled up both the "Brake Pedal Travel" and "Brake Pressure" Now I can't remember the exact values but when you press the brake, lets say it bites at 8mm travel, and goes to 80psi pressure, then the pedal will sink down to lets say 13mm, but the pressure stays at 80psi the whole time, so it's not leaking off. In act, I held the pedal for as long as I could stand and the pressure never decreases.

I went into the BCM in VIDA and operated all the valves and ran the ABS motor to see if there was maybe some trapped air I could free up, but that didn't seem to change anything.

Any ideas?

Thanks everyone

[ShadeTek]

I think you may be suffering from brake pedal creep?

[apersson850]

This is normal for this kind of brake system.

[FIBAMAN]

Originally Posted by apersson850

This is normal for this kind of brake system.

It is probably the servo working correctly. It used to be a method for testing the servo, press the brake pedal then start engine, the pedal should depress slightly if the servo is operating correctly.

[apersson850]

Some of these active systems go all the way to the floor without being broken.