Air pressure in secondary line won't go up to normal level

I have a 2022 Freightliner Cascadia. At around noon today, I was leaving a shipper, and the air pressure on both my primary air line and my secondary air line were taking far too long to go up to the proper levels. At first, it was difficult to move the truck forward because the red brake valve kept popping out because the air pressure was too low. I had to idle the truck for about five minutes to get the air pressure high enough to prevent the red brake valve from popping out on its own. I put the previous sentence in boldfaced font because, to me, the fact that it took five minutes to get the air pressure to a high enough level is proof positive that there was (is?) something wrong with my truck. While I was idling the truck, I pulled the red brake valve out, and I removed both the air lines from my trailer. The air pressure in both the primary air line and the secondary air line still would not go high enough even with the air lines removed from the trailer. So the problem seems to be with the truck, not the trailer. Then I re-attached both air lines to the trailer. And I started driving from the shipper to a truckstop with a repair shop about thirty miles away to get the truck repaired. When I started driving to the repair shop, the air pressure on the primary air line was about 100 psi, and the air pressure on the secondary air line was about 90 psi. For about the first ten minutes that I drove, the air pressure on the primary was about 100, and the air pressure on the secondary was around 90. When I got on the interstat and started driving 65 mph, the air pressure in the primary went up to about 120 psi (which is the normal level), but the air pressure in the secondary only went up to about 110 psi. Then while I was still driving 65 mph, the air pressure in the secondary would intermittently drop to about 90 psi from time to time for no apparent reason.

So I got to the repair shop, and a bobtail truck repowered the load from me. Now I am bobtail. The repair shop is not going to be able to start working on my truck until tomorrow. I always run the truck's engine when I microwave food. After I had been shutdown at the truckstop for a few hours with the engine off, I turned the truck's engine on to microwave some food. After the truck's engine had been on for a few minutes, I looked at the air pressure gauge on the truck, and, lo and behold, both the primary air line and the secondary air line were both at around 120 psi, which is the normal level. So I kept the truck idling longer, and, eventually, the secondary air line pressure dropped from 120 psi to about 100 psi, but the primary air line pressure stayed at 120 psi.

So it looks like my truck magically fixed itself. Did my truck magically fix itself? Why is it the case that when I was at the shipper, it took at least five minutes just to get both the primary and secondary air lines above 75 psi (so that the red brake valve did not pop out on its own), but at the truckstop, both the primary and secondary air lines got up to 120 psi when I idled the truck in just about two minutes?

 

I am not saying that it took five minutes to get the air pressure up to the normal air pressure at 120 psi. I am saying that it took five minutes to get the air pressure up to 75 psi so that the red brake valve would stop popping out on its own. That is uncommon. I have driven dozens of semi-trucks in my career. So I know that that is uncommon.

 

My avatar is a photograph of Marilyn Monroe wearing a potato sack.

 

Right now, my truck's engine is off, and I have the key in the opti-idle position. The pressure in the primary air line is 120 psi, which is normal. The pressure in the secondary air line is about 45 psi.

I have ten years of experience, but I never paid attention to the air pressures with the truck's engine turned off before. Is it normal for the air pressure in the secondary to be as low as 45 psi with the engine turned off?

Here is a photograph that I just took of my air pressure gauge with the truck's engine turned off:

 

Okay. Thank you.