2006 BMW X5 4.4L sport package

When i stop the x5 it seems to roll to a stop and if i push really hard it will stop fully but i have to maintain the hard push or it will roll again. This problem happened overnight also.

Some history: 1 year and 2 months ago had brake fluid changed and brake lines replaced, 6 months ago changed brake pads to cool carbon, problem started 2 weeks ago, flushed brake fluid with 1 liter of pentosin, no air in the lines.

Conditions: i live 12,500 foot up a mountain so dirt roads, steep inclines and declines, country roads

So i have researched quite a bit and i think it might be brake booster or master cylinder. Any test for these to see if bad? Also i want to rebuild the brake calipers also but can only find the seals for the front. Anyone know a place i can get the rear seals. I have checked autohisauz, bav auto, pelican parts.

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2006 BMW X5 4.4l- sports package, premium package
1998 BMW M3 Evolution 3.2l S50B32 CAI, exhaust, tune, (sold)