[cglrcng]

It may be mechanically related also.....Check the manual pressure relief valve by putting your finger over the small hole on the end...Do you feel air leaking out? (If the manual relief valve is set too low it will pop off & begin to leak air and may also keep the compressor running and never actually see 120 PSI. (These are factory set, but sometimes are set too low). Until that compressor actually stops you have no idea how much pressure you really have....start low, then go high per manual pressure relief valve adjustment on the high pressure side.

Next, after turning it above 120 psi (try 125 psi, so you hope the pressure stops building and the compressor shuts off so you can hear for any actual air leaks)....If the compressor shuts off properly at 125 PSI (allow it to hold pressure for a good length of time), a leak down quickly says you need to check each fitting in the system (release the pressure first before attempting the repairs)...were all tubing cuts actually straight and not angled slightly, all fittings using teflon tape where necessary?

Adjusting the manual air release valve is fairly easy, but is done at high system pressure, so slowly & carefully w/ very slight minor degrees of turning after loosening the jam nut only please feel for the air leave at the end hole, stop when leaking is present @ over 122 PSI or more (it will leak down to just above 122 PSI). Tighten the jam nut again.

It should start slightly leaking air out the lil' hole at between 120-125 PSI if adjusted properly (the first inspector will use a jumper between the electrical High Pressure sensor or cutoff switch to test the manual air release valve and temp short/ground the switch). If that manual valve leaks much lower than 120 PSI, it will make the compressor run on as it will never hit the level needed to activate the High Pressure sensor and turn off the compressor due to an actual system created air leak.

Now, once that is set properly, check the wiring on the High Pressure Sensor (is it actually wired correctly?)......Is the compressor now shutting off properly at 120 PSI? Is it holding pressure a long time?

Build from there. Get a high pressure side working (and pressure holding time/test done), to the low pressure regulator and storage tanks and a manual pressure release valve first, before working on the low pressure side ok?