During normal operation a seperate device on the engine side of the firewall called the air governor signals the air dryer to blow off and the compressor unloader valve to stop supplying air pressure to the air dryer at a set pressure of around 120psi. There is a safety valve in the air system called a pressure protection valve usually found on the tank the air dryer feeds(wet tank). This valve senses low air condition(less than 60psi?) and will starve the air governor, horn, wipers, etc. for air. This protection valve is spring loaded and when it sticks the governor will call for more air to be made, and not blow off the air dryer like it should. If your horn or wipers quit working that is the symptom this valve is sticking. I had to take mine off, pour a little ATF in it and work the valve with a small diameter rod. Haven't had overbuilding air issues since doing this.
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