Hello,

My X5 4.8is suffered from the "vacuum pump seal failure, oil in brake booster leading to brake booster failure" issue - I changed the booster with a new unit and re-used my Master cylinder. I got everything back together and no matter how much I bleed it (I'd have bled the system 3-4 times over 4-5 days) I can NOT get the pedal to firm up. It feels very, very soft. I do not have a pressure bleeder, so I am using the pedal to the floor method. Here's what I do:

I have used ISTA-D and done the bleeding with the ABS block as well by cycling the pump - still no go. Pedal soft as hell, goes all the way to the floor.

- Ask my helper to press pedal 5 times
- On the 6th time, hold the pedal to the floor
- Run the "ABS Bleed" procedure in ISTA-D
- I crack the bleed screw (1/4 turn or so) with clear tubing attached to nipple
- Close the bleeder screw
- Ask my helper to release pedal

I repeated this multiple times per corner until no dirty fluid nor bubbles were coming out. I bled in this pattern: Rear Right, Rear Left, Front Right, Front Left.

After doing all this I had absolutely NO firmness return to the pedal; So I thought I wrecked the master cylinder and got a new one and changed it out. After bleeding, there is still NO CHANGE in the pedal feel.

I am out of ideas at this point - I don't believe I have a fluid leak since I'm not losing any fluid (level stable). I am using DOT4 fluid.

The car is parked on flat ground. I have read that jacking up the rear may help but I can't see this helping - the highest point on the master cyl when the car sits flat is the rear port, so any air should be getting pushed out of that?

Never done this before - am I doing something wrong here or is something else broken? Car was FINE before I started this (with the exception of a broken brake booster making the pedal super hard).