The air throttle control is called a treadle valve, it is 30 years since I worked with these but it is a plunger operated valve similar to a foot brake valve. It works on vehicle air pressure. You could take it apart as it will consist of O rings (take pictures as you strip it) it may be an easy fix. Google the make of valve and you may find a replacement or repair kit. Good luck, remember to keep us posted on your progress.
Update, have a look here as explains how valve works and safe removal of valve.
http://www.oshkoshmanuals.com/litera...ies/625-3A.pdf