MMmmmmmm........ I have an older semi trailer that has no spring brakes, but it does have an air parking system. It works by a different air valve. The valve that applies the service brake IS the emergency valve. If air from the emergency/supply goes away (IE setting the parking brake), it sends air from the tank to the air can, like you are appling the brake. And that's how it parks. But if all the air bleeds out, the brakes release....
I doubt a truck would have the same system.
Does the rear axle air cans have one or two hoses going to each can? If there are two, then it "should" have a spring brake set up.
