The insides of the ABS pump is no exception. Brake fluid is hygroscopic (meaning it absorbs moisture from the air) and while the brake hydraulic system is closed it is not air tight. This is why BMW and most other manufacturers recommend a complete exchange of the fluid in the system
bi-annually, every 30,000 miles, or when fluid moisture reaches 3% or greater.
Moisture carried into the pump through the fluid attacks the dissimilar metals used within and galvanic corrosion starts to occur. The release of ions forms a gel like sludge which in turns congeals in the narrow passageways inside the pump and builds up on the pre and post solenoid filter screens. Over time this build up clogs and outright obstructs flow through the pump hindering ABS function, and normal braking performance. Removal of this build up is not possible by flushing the system with fluid as you would as a routine service and requires complete tear down of the pump.
A pair of shuttle valves are driven by a cam lobe on the end of the DC motor integral to the pump which is what builds additional brake system pressure in an ABS triggered event while you stand on the brake pedal hoping the system works when you need it most and every second counts These shuttle valves are essentially miniature brake master cylinders and one of the most common failures is they seize in their respective bores and the return springs snap rendering the system largely non functional, or worse... a caliper binds on/clamped causing potential loss of control.
Strictly speaking almost every single pump I've torn down has been unfit for service with significant clogging of pre/post solenoid filter screens, broken springs and or seized shuttle valves. Rarely is there any sort of electrical failing which is really all the ABS computer in the cabin can "see"
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