Trailer Brake Functionality

Hello, I am not a truck driver. I have a class B CDL and I work in fleet service shop turning wrenches on everything from backhoes, fire trucks, police cars, ambulances and regular civilian transportation. The question I have is whether or not a trailer on tractor trailer set-up has to have parking brake chambers. According to DOT , in 1975 and newer trailers had to have a parking brake system. The truck has parking brakes on it's rear axle. Is there another system that trailers use to apply a parking brake when the trailer is disconnected? This 5th wheel boxed trailer is at least 25 years old. The entire tractor-trailer is about 80 foot long. On the trailer, it has service brake chambers on the single rear axle, but no parking brake chambers. There is a red airline and a blue airline connecting the older international 7 series truck to the trailer. NOTE: This is a Hazardous Materials truck purchased by the city for our fire department decades ago. What I have found on firetrucks and their kin is that they do not follow any standards of the regular civilian world of vehicles (they are kind of built like a motor home or a Johnny Cash car) in their designs and do not follow any service manuals for International or for anything else for that matter. Emergency vehicles in Virginia are excluded from several laws, such as passing emissions for example. Is there some trailer protection valves that operate the service brake chambers when a trailer is disconnected(there are no parking brake chambers on the trailer)??? The trailer does have dual air tanks above the trailer axle, but they appear to feed the air bag suspension for the trailer axle via 2 height sensing valves. Maybe these tanks somehow also feed the service brake chambers when the trailer is disconnected? The trailer is very low to the ground and you have to crab crawl on your back to investigate the system. Any thoughts or knowledge would be appreciated. Currently at home... sorry I didn't write down all of the info on the truck before I left work. Also it has not been started for over 2 weeks... waiting on engine oil drain plug that is on back order, was doing a PM and someone had previously rounded of the flats, got out with vise grips... so no oil is in the engine. Yes, we have issues getting parts just like everyone else. Once I get oil back in the truck I can start the engine, build some air pressure and then I can do some tests that don't involve me trying to tap into the air system with an external air source. For the PM, I needed to change 6 of the tires, because NFPA 1910 says 7 years or less on the date code for fire apparatus. Front steers were 15 years old and when I went to change the 4 trailer tires (they were 8 years old)... I realized the rear tires would spin after jacking them up and putting the axle on jack stands. The rear tires on the truck have their parking brake applied(plenty of room to crawl under and visualize on the truck's dual service/parking chambers). Yes... I had the other tires that were touching the ground scotched with wheel chucks and the truck is outside on level ground. Safety first.

First really dumb question, did someone cage the brake chambers?

the trailer brake system is supposed to automatically apply the brakes when the psi of the system is too low. This is the "parking brake" system.

i like to start with the easiest and most obvious in troubleshooting, because I don't like to get filthy crawling around under there.

More info needed, pic would help. Double can or single, cammed over rod many variables.