Are there any simple tests you can carry out to check servo valves?

If the cylinder moves then there is every chance that the servo valve is OK. This simple test only shows the valve is basically functional, it will not show some of the more subtle faults the valve can have. If you get no movement then the valve is probably faulty, at this point check pressure is present at the “P” port. If you have more than 5 machines buy a servo valve tester (try www.blow-moulding-controls.com). Simply plug it in and the valve and cylinder should move.

Some servo valves can be accidentally fitted 180 degrees out of position (Moog 62, 76 and 760 series for example). If the hoses have been off for any reason check they are the correct way round. If the pressure is on the correct port of the servo valve and the cylinder rod is not seized (very unlikely) then the servo valve is probably faulty.