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

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2003 E39 530i
1992 MX5
2005 E53 x5 4.8is (sold)
1990 K75S (sold)