Panic Braking: Pedal almost to the floor, anyone experienced this?

[cn90]

OK,

Today it was snowing and icy. During a hard brake at 20mph (no accident or anything, just going down the hill) with foot firm on brake pedal, the ABS was activated and about 6-7 sec later the pedal goes down further, basically only 2-3 inches from the floor.

On dry road, there is no brake pedal sinking.

Are there some valves in the ABS Hydraulic Module that allow fluid to "flow around"?

I know the Master Cylinder is good b/c on dry road, the brake works just fine.

[jdstrickland]

Yes. It's normal. Mash the pedal because that's what you do. ABS will detect the resulting wheel lock and take over operation of the braking system. YOUR JOB IS TO HOLD THE PEDAL ON THE FLOOR while the ABS makes machine gun sounds.

[cn90]

What I am curious about is: why the brake pedal sinks after 6-7 sec. of sustained pressure by my foot?

I also own a 1998 528i and 2 Volvos at home, these other cars don't behave like this (i.e. no pedal sinking with sustained braking on ice).

I wonder if the E53 ABS Hydraulic actuator has a "secret channel" where extra brake fluid is channeled into, creating a brake pedal sinking effect.

[admranger]

I've heard of an "ice" program in the ABS but that usually pushes the pedal "up" not what you're describing.

Knowing your level of thoroughness from reading your DIY threads, I'm going to hazard a guess that the fluid is reasonably new and the brake system was properly bled. Frankly, the only other thing I can think of is some sort of a master cylinder issue during ABS braking.

[David.X5]

There is an abs pump. Just a guess, but perhaps yours is not working and after several seconds of abs activation it has used all the pressure available from the pedal and is lacking the extra capacity to keep the pressure up as intended.

[cn90]

Brake fluid is good and all brake pads/hoses are in good shape. I maintain my cars meticulously...lol.

- As I mentioned: dry road braking is fine, zero issues. If I have problem with Master Cylinder, it would happen at any temp, no matter what.
Plus, on icy road, once the brake locks up and ABS starts working, there is very little heat built up in the pads.

- I wonder if someone in forum can do me a favor, find an icy empty parking lot (Target, Walmart at night with no cars, (or empty road with no traffic), drive around 10-15 mph and apply sustained brake for 6-7 sec as if it is panic braking and post your response: any pedal sinking or not. I will send Santa Claus to you...

[David.X5]

I will do the test when I have a chance. Record Christmas temps here - 71F !

[upallnight]

Did you ever do any brake work such as replace a caliper and some of the fluid drain out of the system? I don't think that you can bleed the abs system when you bleed the regular brake system, but if the system was open, fluid can be drained out of the abs system. You will need software to operate the abs pump to bleed it correctly.