Now thinking further about it the way ABS work is is if a wheel is about to lock up the valve opens and releases the pressure from the corresponding line, then closes and pump brings the pressure back up. If the brake pedal is released all the brake fluid would just go back to fluid reservoir through the main brake cylinder without pressurizing even if pump was running constantly.
And it cannot be main brake cylinder at fault as well as it has 2 diagonal circuits so if that was sticking if would affect one front and one rear wheel not both rear wheels.
It's got to be mechanical issue within calipers or handbrake cables.
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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