please check for kinks in the metal line, all the way to the ABS pump.

try tapping the pump head with a mallet, be careful with it though.

If you remove a line - you can easily purge out the air from the pump with a powerbleeder. Hopefully you have been on top of brake fluid flushes every 1 or 2 years and the pump is not corroded internally.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)