Update: So I went ahead and cut the booster out of the circuit by removing the check valve/line. Then I capped that.

After starting vacuum climbs very quickly (2-3 seconds) to 27in Hg as compared to climbing very slowly (90 seconds to 25in Hg) when the booster is in the circuit.

I'm going to call it a failed brake booster. Ordering replacement parts now. If it's not one thing, it's another with this thing. Does not want to leave.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)