Hello. I own a 2017 Tahoe. My issue is that the rear air ride suspension is stiff and the ride is extremely bumpy. I took it to a mechanic who I trust and he found only a communication code that came up. He was able to control the air suspension with the diagnostic device, meaning he was able to lower and raise the backend. He lowered the backend through the device BUT when he started the engine it slowly filled up. They test drove the car and could feel the suspension get stiffer as they drove. Eventually it filled up. Clearly its not a a relay or a compressor. Any ideas?