brake pedal in one of my cars (not a bimmer) drops to the floor when you nail it. the system has no leaks and is full of fluid as well as flushed.

I'm guessing there is something up with the master cylinder...any body have any thoughts?

thanks,
Ron

If the pedal sinks to the floor when steady pressure is held on the pedal one of three problems is present; (a) there's a leak at a caliper or line to a caliper, (b) the master cylinder is bad, (c) there's a fault in the ABS module. If the case of (a) or (b) fluid will be leaking out somewhere and after several cycles you should see the fluid level in the reservoir drop. That won't happen for (c) as the fluid is merely circulating within the system.

the seals in the mastercylinder could be leaking. you wouldn't see an external leak if it's internally leaking.

thanks for the input guys, there are no leaks that I can find. everything is dry around the seals, calipers and lines.

the pedal doesn't sink after holding it, it just has an excessive amount of pedal travel and the pedal needs to be pressed 3-4 inches before the brakes are effective. at that point, the pedal is near the floor and can actually be pressed into pedal stop without any effort.

Sounds like air in the system or a bad mastercyl.

This the ML?

yep, on the ML430. I'll rebleed the system again today.

Chiming in for a similar problem. The pedal sinks almost imperceptably if I press firmly at a stop, but the big thing I noticed is that after rebuilding all four calipers and bleeding /cycling ABS / bleeding (repeat), I still get initial lockup and ABS activation on my Right Front wheel before any of the others. (had that problem for a while and a sticking left front, thus the rebuild)

I don't know the circuit layout, but I'm assuming that the fronts are on separate circuits, so I reasoned that I've got one seal going bad giving a lower effective pressure to my left front. Not much at all, but just enough.

I figure a new MC is the next step, but I have other financial obligations before I can do that. Curious to hear what other responses are.