Line failure? Something is preventing any pressure from being built up in both circuits though, so maybe it could be the master. I've had hard lines under the car (in an E53 corrode and eventually split under pressure, but unless a line in both circuits went at the same time, it seems unlikely you would loose any and all brake function..

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~Scott

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Black Sapphire Metallic/Sand Beige '13 E70 35d, SP, NAV, Tech, Comfort Access/Seats
20" Style 214's Wrapped in Continental DWS06 Extreme Contacts

(SOLD) Alpine White/Sand Beige '06 E53 3.0, SP, PP, CC, RC, NAV, Sat Radio
19" Style 63's Wrapped in Bridgestone Dueler HP Sports-- Summer Shoes
18" Style 131's Wrapped in 255/55-18 Nokian WRG2 SUV's -- Winter Boots