The step cover is leaking air from some where and it looks like a hard thing to work on. I just finish changing out the norgren
valves that were leaking but the biggest leak comes from the step mechanism.
You can put the step cover out with the door open. Check the cylinder rod for hair, dirt, and lint. If it's dirty, try cleaning the rod and cylinder end. Run it in/out a few times with a cleaning each cycle. Ours had a profound leak at the rod seal/wiper and cleaning cured it. At 20+ years old, the rod wiper gets hard and ineffective, allowing debris to compromise the seal. Apparently, some of that was getting between the rod and seal, causing the leak. On our coach, the step cover cylinder is a throw away or I would have rebuilt it.
Greg
I found the PRV was leaking so I have change it and that has slowed the leak considerably.