That has to be frustrating as heck...

So I guess the problem continuing suggests the brake booster. RMeuropean will get it to you very quickly and the price was pretty fair (no affiliation, just a happy customer).

Yes, love the service at rmeuropean! Get all my parts there. Great prices and free overnight shipping. Can't beat it.

I installed the new brake booster and changed the three heater hoses that terminate at the firewall "while I was in there".

The project would have gone a lot faster if I hadn't dropped so many things into the nether regions of the engine...getting pretty good with the flexible part retriever (claw tool).

Tomorrow I will do the GT1 pressure bleeding of the brakes and bleed the coolant system. I need to go search for the procedure again (GT1 steps). I always get lost in the menus of the system. Hopefully google will come through for me tonight!

I have a nice pdf'd writeup for the bleed procedure for my E36. Similar for these - it'll get you to the right spot a little quicker :). PM me your email if you want it as the file is too big to post.

Got it bled using the GT1 software. Used a quite a bit of fluid to get everything out.

Prior to using the GT1 software I hooked up the brake bleeder (Motive pressure bleeder) and cracked the hard lines to the master cylinder open. Got some air out that way first, then bled per the GT1 instructions. Easy peasy.

Any update ?
What fixed your brake pedal in the end?

So there is a slight chance that it was air caught somewhere in the system,
and when the dealer activated the ABS via their brake bleed service procedure it eliminated the increased pedal travel ,
coincidentally after you replaced the booster?

I have same sort of problem;

Like you, I have replaced the master cylinder with no improvement;
but it seems you were so unlucky that you replaced your booster and by coincidence the replacement booster was bad as well, and the fault was in same manner as your previous booster;
I'm reading of another X5 owner with the same problem, BMW just replaced his booster and the problem remains-
Hmmm... what should I try next?
The only possibilities left for me are:
1. Somehow there is still air in the lines (but it doesn't really feel like air, I get max braking once engages).
2. Pad knock back (causing increased pedal travel)
3. Non-return valve (or valves) faulty, and or hard vacuum lines.
4. Brake booster.

Your experience is making me think I should just bite the bullet and replace booster,
(I've got a 2nd hand one,
I'd have to wait ages for a new one to come to the land of Oz from overseas),
But I'm trying to nut out the problem and narrow it down before ripping out everything again,
Man I'm ripping my hair out! :angryfire:pullhair:

Good timing on this thread. My 4.6is is having these issues.

For those trying to sort out booster vs other issues. I would remind that the X brakes have two separate systems at work here. The hydraulic system: which is the master cylinder, lines out to brakes etc. The other is the booster side: vacuum from the engine (intake or vacuum pump, 1 way valve, and booster (big diaphragm). The hydraulic side should "feel" solid pedal, not spongy when pushed. You can test the hydraulic side with the motor off as after 2-3 pedal pushes the vacuum booster will be out of residual boost. With the motor on you can test the booster: all it does is help you depress the pedal.

So:
1. soft or spongy pedal = issue on the hydraulic side (probably air in the brake lines). X system is hard to bleed after master cyclinder change: for me it took running the procedure on GT1 that activates the abs pump twice.

2. pedal firm but very hard to press: like it takes 2 feet to stomp on pedal even while engine is running. Maybe revving engine in neutral helps pedal be better. This is a booster issue. Common on the V8 BMWs: few tsbs by BMWs and limited recall on 4.8 motor but did not include many X5s. If your find oil in your vacuum line past the 1way valve then it's likely in your booster and the booster will fail.

Hope this helps and best of luck.

J