Phil, this could be brake fade.
If a lot of braking occurs ie. going down hill and reducing speed using the brakes constantly, or the shoes are over adjusted and binding the drums get so hot they expand away from the shoes, this results in the pedal having an excess of travel and the brakes not being that efficient.
This was a regular occurance on fire engines of yesteryear and was quite scary if you were not aware of the situation. Many a time a new driver would send the engine up with a defect on the brakes but this was a natural occurance when you had been driving hard on the way to a call.
If the drum was so hot that it had melted the dust cover this could be the problem.
Hope this helps in some small way.....
Rob...
