Just as hydraulic systems have pumps, most pneumatic systems have a compressor of some sort. In a turbine engined aircraft its usually the engine itself that provides engine bleed air.
A hydraulic system needs a reservoir of fluid, if you get a leak and all the hydraulic fluid is gone, its gone. End of story. With pneumatics there is usually an inexhaustable supply of 'fluid.' As long as any leak is within reasonable limits the pneumatic system still works. In fact most operate on a "total loss" principle anyway, used air is exhausted.
Both systems have their uses and most large aircraft have both. (Although such a thing probably exists somewhere, I've yet to see an aircraft with a hydraulic engine start system!)
By the way, check out where the residual pressure in hydraulic systems comes from. (As a clue - you're breathing some right now)
Cheers m'dear
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Through difficulties to the cinema
[This message has been edited by Blacksheep (edited 14 November 2000).]