The leveling valve isn't holding pressure correctly, But unless you have a load in the rear of the car the system shouldn't need to hold the car up. The system is designed to compensate for loads in the rear of the car. If it's working all of the time the rear springs are sagging. This will cause a lot of expensive problems. Springs are cheap!! & many times spring perch pads can be installed in thicker sizes to bring the ride height back to the correct level.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR