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The only way to find a leak is to turn off the truck, let both sides settle to the "kneel" height, then get under the truck and listen for air leaking.
Also, check and see if the problem is that the passenger side isn't deflating properly...therefore causing you to think the drivers' side is the problem.
If that isn't the culprit, then check the air connections on the drivers' side and the condition of that air bag (dry rotting, tears, cuts, etc).
When you are driving, do you here the compressor come on a lot or just initially?