My sons "9641 Pneumatics Add on Set" now suffers from stiction.
How can this be fixed? Can it be cleaned? Is there a recommended lubricant?
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In this discussion at LUGNET the conclusion seems to be that you could use dry graphite if you are comfortable with graphite powder everywhere. Else you should try lithium grease or canola oil.
There is also a serious warning about using hydrocarbon oils that could damage the rubber seals inside pistons and the rubber air tubes.
One poster also noted that stiff pneumatic cylinders can be unstuck by moving them by hand for a while. This definitely does worth a shot before trying to add any kind of "foreign" material such as oil.