After scratching our heads and doing a thorough check over the truck that had brake problems last week, Ciaran and I found the cause. There is a fitting in the air lines for the rear brakes that is supposed to vent pressurized air when the brake pedal is released, therefore allowing the brakes to stop stopping the truck. Turns out that this vent had been completely blocked by the nest of a mud wasp. All it took was a screwdriver and some WD40 to clear the mud nest and allow the air to escape and the brakes worked perfectly.
A 15,000 pound truck had literally been brought to a screeching halt by an insect about thirty-five million times smaller than it.