Changed Master Cylinder. Pedal still going to floor?!
 

So 6 months or so ago I was frustrated by something in life and I vented my frustration sitting in the driveway by mashing down my brake pedal as hard as I could in a childlike tantrum fashion. Something I'd normally not admit on a forum LOL. But it was the foundation for the problem I'm having.

A mile up the road I noticed the brakes were feeling spongey.

I turned around to go home slowly and safely with half working brakes and by the time I got home the pedal was just going to the floor. At the fully-down position it would still hold my brakes fine to maneuver the vehicle in and out of the garage.

I figured it blew a seal in the 15 year old Master Cylinder and I finally got around to replacing the master cylinder a few weeks ago, and finally had time to bleed the brakes today.

During bleeding of the Front brakes I noticed that the pedal has 2 stages. The first half of travel, a kind of "click," and the second half of travel down to the floor.

When all brakes were fully bled, engine off, I thought it was weird that the pedal wouldn't harden. I could travel the pedal halfway down to just about where that "click" might happen, and then it was hard after that and wouldn't go to the floor.

I re-bled the brakes in case I missed some air. Nope no change.

So I thought I'd take it for a test drive and see how the brakes acted.

I started the car up and pedal went down to the floor exactly the way it had felt before I replaced the master cylinder!

So clearly the MC was not the culprit. But I'm not familiar with BMW's so I have no idea what else could possibly cause the pedal to hit the floor??? Zero resistance, just poof on the floor.

Brake fluid remains full in the reservoir and I don't see any fluid anywhere on the ground so I don't think it's dumping fluid anywhere. After bleeding I had pressure in the brake system. It was only after starting it again that it hit the floor.

The fact that the first stage of Master Cylinder travel never hardened up leads me to believe it's a problem in the rear half of the brake system perhaps?

Any input would be much appreciated. Thanks!

You still have air in the system.

Best is to press the brake pedal all the way down (engine off) and hold it there for a good 30 sec or more.
This way any air trapped in the MC can travel upward back into the reservoir.

Please update the thread if it works or not.

I'll try it shortly but don't have high hopes.

The pedal went from having some pressure and resistance after bleeding to immediately smacking down to the floor as soon as I started the engine, exactly as it has done before the repair.

I'm wondering if it's the ABS Pump? I know on some Ford's that a seal blowing out of the ABS Pump can cause the pedal to go to the floor.

Maybe something the booster assembly, since the issue shows up once there is vacuum...? :dunno:

You got air in the ABS pump. Take it to a dealer or an indy who has the equipment and knowledge on how to bleed the system.

I have INPA, and my understanding is that I can use it to bleed the ABS system. But for some reason I can connect to every module except DSC to activate the ABS pump. Don't really want to pay for a dealer to spend hundreds on something so simple. I built GM race car engines for 12 years so I'll be damned if I let brake bleeding get the better of me LOL. Now I have to figure out why INPA won't connect to the DSC...

Is your abs functional? No trifecta?

Do you need to search for DSC/ABS in German? I'm sure somebody here knows. I use foxwell scanner for exactly the problem you are having

E53's can be surprisingly difficult to bleed. You may need all three:

INPA method to get air out of ABS module
Pressure bleeder attached to reservoir
two-man pedal pumping bleed

A combo of the last two - pump/hold pedal while an assistant opens closes bleeder screw WHILE the pressure bleeder is attached to reservoir is quite effective.

No errors or lights indicating ABS issues.

I ordered a Foxwell to arrive today.

Just ordered a Motive pressure bleeder as well that arrives tomorrow.

Not sure how much time I'll have this week to finish the job, usually Tuesdays is my day to work on cars.

Will report back!

+1 ^
The trick is to pressurize the brake system to 20psi, crack open the RR caliper bleeder screw, bleed until zero bubbles (can take 10 mins), activate the ABS MODULE, press on the brakes 5x, continue to bleed to flow out the air that was trapped in the ABS block, repeat the ABS and manual brake activation until everything is clear and free of bubbles.
Move to LR, RF and then LF. Road test
Rebleed and you should be good to go.
Air seems to like being trapped in the RR and RF channels for some unknown reason. :)