You could also add parking brakes to another axle (tandem drive) by changing the brake chambers to doulbles instead of single. I did that on two trucks I bought this winter. One had a cable parking brake. We put 4 chambers on it. The other had spring chambers on the front tandem axle but not the rear, so we put 2 chambers on the rear one.However, not all axles lend themselves to this. The local shop that did it tried to put spring chambers on 2 other trucks and the design prevented it from working. Because of how the brake chambers were located if they went to the longer spring type they would hit. | 
Another air brake question
