About that black plastic unloader pipe, it connects into the air system pipe work, not quite sure how but as I understand it, when up to pressure it lifts the valve in the compressors head so it runs light when up to pressure, to save power, fuel and wear.
When I fitted the Cummins into Forceful it was all too complicate to incorporate this, due to confusing the original unloader valve on the air tank, so I just connected the compressor discharge pipe as per original, so now the compressor runs full time like the original and excess air is wasted to atmosphere through the tank valve.
These compressors can produce a lot of carbon which can block the discharge pipes etc. so best to have a look before connecting up.
No problems so far, just my way of doing as fewer mods as possible, keeping it simple..
I have a manual for Cummins L10 engines and many things are similar, if you need a scan of relevant pages just ask.