Most hydraulic or pneumatic faults are caused by dirt. Very small particles nick seals, abrade surfaces, block orifices and cause valve spools to jam. In hydraulic and pneumatic systems cleanliness is next to Godliness. Dismantling a valve in an area covered in swarf or wiping the spool on an old rag kept in an overall pocket does more harm than good.
Ideally components should not be dismantled in the usual dirty conditions found on site, but returned to a clean workshop equip ped with metal-topped benches. Too often one bench is used also for general mechanical work: it needs little imagination to envisage the harm metal filings can do inside a pneumatic or hydraulic system.
Components and hoses come from manufacturers with all orifices sealed with plastic plugs to prevent dirt ingress during transit.
These should be left in during storage and only removed at the last possible moment.
Filters exist to remove dirt particles, but only work until they are clogged. A dirty filter bypasses air or fluid, and can even make matters worse by holding dirt particles then releasing them as one large collection. Filters should be regularly checked and cleaned or changed (depending on the design) when required.
Oil condition in a hydraulic system is also crucial in maintaining reliability. Oil which is dirty, oxidised or contaminated with water forms a sticky gummy sludge, which blocks small orifices and causes pilot spools to jam. Oil condition should be regularly checked and suspect oil changed before problems develop.