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.