Look at the brake cans on the rear axle.
If you have three air lines going to the cans, you have DD3 brakes.
If you have two air lines, you've got spring brakes.
Maxi brakes is a generic term for these brakes, similar to how Kleenex is now a generic term.
There is a ton of information about brakes on Bendix's website, well worth snooping around for awhile under the service & support tab. Even have tutorials about your air compressor! www.bendix.com
FWIW & HTH. . .