Governor signal line

The air in the signal line holds open the unloader valve when the pressure is high enough, about 120 -130psi. That way air is not pumped to the tanks when it is not needed. Once the pressure drops to about 100 the air is shut off allowing the unloader valve to close and the compressor to pump air to the tanks.

Constant purging - these may vary depending on the make and model of dryer. What do you have?
Excessive leakage
Defectjve delivery check valve
Defective governor
Compressor unloader leaking excessively

 

A pic of the governor and air dryer would make helping you easier.

 

Below provides more detail.
"Is there always supposed to be air in signal line? " No

As the system reservoir air pressure drops to the cut-in setting of the governor, the force exerted by the air pressure on the piston will be reduced so that the pressure setting spring will move the piston down. The inlet valve will close and the exhaust will open. With the exhaust open, the air in the unloader line will escape back through the piston, through the exhaust stem and out the exhaust port

 

Very likely will. The older Pete's are having the same issues, but running 20-30 year old stratoflex. You can spray down the supply line to the governor and signal to the dryer with soapy water, and the both lines will be leaking the entire length.

Whether stratoflex or plastic, I usually replace both with high quality plastic line, and don't use quick connect fittings at any of those connections.