Most hydraulic and pneumatic faults are caused by dirt. Very small particles can nick seals, abrade surfaces, block orifices and cause valve spools to jam. Ideally components should not be dismantled in the usual dirty conditions found on site. These should be taken to a clean workshop equipped with proper workbenches. Components and hoses come from manufacturers with their orifices sealed with plastic plugs, to prevent dirt ingress during transit. These should be left as they are during storage and removed only when the component is to be put to use.
Filters are meant to remove dirt particles, but only work until they are clogged. A dirty filter may cause the fluid to bypass and can make things far more worse by accumulating the particles and then releasing them all in one lump. Filters should be regularly checked and cleaned or changed when required.
The oil condition in a hydraulic system is also crucial for maintaining reliability. Oil, which is dirty, oxidized or contaminated, forms a sticky gummy sludge which blocks small orifices and causes the valve spools to jam. The oil condition should be regularly checked and the suspect oil changed before problems develop.