HELP 2007 Peterbilt endless air dryer cycling

Replaced air dryer cause it leaking though the purge valve spitter endless cycling. Here is what happening (c15 cat) air will build up to 130 air dryer purge and slowly leak air out the purge valve down to 110 compressor kicks in back to 130 purge just a endless cycle. The new dryer is the same as the old and we went got another purge valve still the same thing. But if you turn the motor off it will hold 130 for two days. Key on with out engine running will hold air. The only way this dryer and the old one leaks air is with the engine running. Also the new dryer came with a new governor. What is cause of this leak down only with the engine running???????????????? Replace air line to governor also.
Little more back round it a 379 air dryer sit outside the frame behind left steering tire and has 405k miles

 

ok thanks will check it out in the morning and report back

 

Replaced the unloaders but still leaking down replacing the compressor Monday (can't get one before then) that should fix it. Something is not right with it but it is 14 years old.

 

Yes it is the head been replaced before. Because of age we just went a whole new compressor

 

Well we changed the compressor and that was not the problem . After much searching we tried hooking up the computer to the ecm and turning on the fan cultch with the computer and found the fan hub had a big air leak. We couldn't hear the leak with the engine running see the ecm only supply air to the fan hub once the motor has run for 30 seconds. This air leak is what is causing it to cycle all the time.