Ed,
I am guessing, since you don't have a bleed valve on that contraption, that you're using an air compressor other than your coach's air.
For others, like myself, who have built our own based on a youtube video, and using the coach air system, as the pressure goes below the target tire pressure you simply flip the bleed switch for a couple seconds which takes the coach air pressure down below its start up pressure, it then starts up, and after it again goes above your target pressure, you can continue filling the tires.
There are actually 2 youtube videos, the original didn't have this feature and required 2 people to fill the air or a single person willing to hop in the coach and press the air brakes to bleed the pressure.
Here's the link to the 2nd one
https://www.youtube.com/watch?v=n5fK54DKWP0Mike