Contamination of the hydraulic fluid in a hydraulic system prior to the first flushing. It comprises the contamination that is in the hydraulic fluid itself prior to filling the hydraulic system and the particles of dirt from manufacture that have not been washed out of the individual devices adequately prior to installation ("built-in dirt"). It is imperative this dirt is filtered out at the start by flushing the installation. For this purpose a flushing system with a dedicated pump and filters is often used.
Fluidlexikon