The pedal slowly sinking to the floor is brake fluid leaking around the piston in the master cylinder. The fix is to replace the master cylinder. It’s not dangerous, just annoying.

A soft pedal feel or a spongy pedal is from air in the system. Anytime the hydraulic system is opened, you’ll introduce air into the system. Air is compressible while fluid is not. You’re feeling the air bubbles compress. This is not dangerous if there’s not a lot of air, just annoying and reduces the braking quality. Air can be in the slave cylinders or in the abs maze. The fix is to run the purge program using a computer tool.

It sounds like you had multiple brake issues and you fixed the booster issue. Now you have to fix the other two issues.

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2005 X5 4.4i SterlingGrau, Sport Package, DSP
Build 3/12/2005