2004 GMC Envoy Rear Suspension Load Levelor

Sometimes over night - my rear suspension lowers way down. When I start the car in the morning - the air pump works and puts the rear end back to level.
Obviously I have some kind of leak.
Can you tell me how to find the leak - maybe it's just a hose or something - hate to take it to a dealer as they'll probably want to install new pillow shocks etc.
It must be small leak cause it takes most of the night to leak out. Parked at work most of the day - it only lowers a little.
thx Mike