'06 4.8is acquired - need some guidance

[mightydave]

Hey All -

Recently acquired an '06 4.8is after about a year without an E53 in my life. Fun to be back. This one had a known braking issue when I bought it - previous owner was certain it was a failed brake booster. He had just replaced the vacuum pump, hoping it'd resolve the issue - but it didn't.

Here are the symptoms I experienced when it arrived last weekend:
* Incredibly firm pedal, but inconsistent
* Virtually no stopping power (definitely not drivable)
* After a few pumps, pedal would essentially go to the floor (with some resistance)

First thing I did was pull the covers under the driver's side. Both hard brake lines under there had perished. Rust and active heavy leaking. I was stoked as I didn't really want to do the brake booster (which he sent with the X5).

I did the repair yesterday. Both lines - one original union was used and one was replaced. Used a pressure bleeder to pressurize the system, no leaks, everything was happy. Went through a normal bleeding procedure with the pressure bleeder - essentially doing a full flush while I was at it. Lots of air in the rear, not too bad in the fronts. Afterward, pedal gives very little resistance up top, and bottoms out every time. No stopping power.

This morning I did the ABS activation using my Foxwell tool - walked through the steps where it activates the pump and you move through each corner. In the right rear, I saw some bubbles, but not a ton. In the left rear I saw some old fluid come through. Fronts were good.

Afterward - same experience. No stopping power, pedal is not firm. When the system is pressurized with the pressure bleeder, the pedal is perfect.

While doing the repair, I kept adding to the reservoir but I'm concerned it ran dry at some point. That has me concerned about the master cylinder.

Given all that, what would everyone recommend as a next step? Bite the bullet and do the booster? Pull the MC and bench bleed it (never done this)? Do a few more rounds of bleeding using the Foxwell tool?

For fun, here's a shot of the state of the lines when it arrived last Sunday.