My GL450 driver rear air bag is low?

What is your question?

When I first get in and start my car, the rear is low and rough pulling out of garage. I wait and it builds up pressure like on a bus and is better. I've had this problem before a long time ago. If the car sits for any time, I see it is low again. Is it a value or strut or airbag? I do not want to be taken advantage of at the repair shop. Want to know what the possibilities are before and the possible cost of the repairs to compare to. Thanks