By the cable being 'loose' I will assume you mean that the cable remains attached but falls slack in the middle of the run.

Rear brake springs are not able to overcome friction and pull the cable tight when brake lever is released.
Either find the source of friction and reduce it, or increase the strength of the brake springs.



Check that there are no kinks in the cable and housings, make sure that the ends of the housings are cut and ground flat underneath the ferrule end caps, also see if any loops are excessively long while doing this, lubricate inside of housings with triflow before reinstalling cables.

Check brake arms for springiness when no cable is connected, if they are weak even without having to pull cable, either rebend the springs, or reset the spring position to a higher one (under the brake mount are 3 spring mounting holes)

Strongly recommend based on the issues you are having that these bikes be taken to a good LBS and given tuneup/safety check before allowing paying customers to ride them. Injury liability from impropor assembly can easily kill your tour business.