Cummins air compressor

Probably the line from the tank to the governor so it knows when to kick in and out the air compressor.
If it has those cheap plastic push-to-connect fittings. Take them off an put on brass compression fittings, usually fixes it.

 

Tis the unload line. When cut out pressure is reached the governor sends air to the compressor. Valves open and compressor stops building.

 

The purge valve on the air dryer is probably toast the o-rings go bad and come apart and get stuck in the valve not letting it close completely new purge valve is like 30 bucks for the kit new o-rings lube and heater if it is an ad-9 system

 

Mine has 3 3/8 bolts holding the purge valve in the bottom of the dryer unscrew those take a small flat head gently pry out the old one make sure all the prongs come out they like to stick inside the air dryer housing assemble new purge valve lube up the o-rings on new one slide it back in place put bolts in and plug in the heater at least that’s how mine goes but my dryer is in a very accessible spot on my frame behind passenger steer tire on my cabover good luck man 122 bucks now oh how the world is changing