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I think you are going to need to look at some truck suppliers for air wiper parts. The air wiper valves are known to leak, I had an 86 MCI that had air wipers and just like you said, I had one valve that leaked and you could fiddle with it and get it to stop. Basically they need to be rebuilt or replaced. Like others said you can pull them apart and see what is inside for replacement. Sprague is a fairly common brand for the air wiper valve. 

http://www.rometruckparts.com/pdfs/techi...Motors.pdf

http://www.rometruckparts.com/pdfs/contr...series.pdf