Hi Alan,
First of all remove the wheel. Inspect the braking mechanism inside the wheel. Clean it and eventually sand the braking shoe (is that the right word?). If your braking cable is broken, this is easy to see at that moment. At the wheel there is the braking lever and a adjustment screw. Adjust them both in such a way that the air brakes can fully extend, but meanwhile have enough braking force on the wheel. It's quite some fiddling and you need a second pair of hands.
Tip: If you are a heavy guy, you could consider to lessen the brake to avoid the 15 standing on it's nose with a full stop. It saves you some polishing in the winter
