Hi. I know this is a really old thread, but I'm having the same issue......sort-of. What instantly caught my eye is that I have the same compressor......sort-of
Mine is its bigger, beefier brother, with TWO motors on it, but aside from that, it's the same brand and I'm positive uses the same parts. I was curious about how the unloader valve on the pressure switch works, since you seem to have fiddled with it. The part I'm mostly interested in is in post #11.
While I haven't actually modified mine in any way, it too began slowly losing pressure overnight, which causes it to start up on its own, wasting power and wearing out the motors unnecessarily. I started by replacing one of the leaky quick connect fittings on the pressure switch outlets (mine has two) and that did improve things slightly, but it's still not perfect, so further digging was required.
One thing I find interesting is the way they implemented the dual motor situation when it comes to the discharge procedure: each motor obviously has its own check valve which connects the heads to the tank itself. The check valves are the kind that have a second, much smaller port Teeing off on one side, and that's connected via the aluminum tube to the pressure switch's unloader valve. In this case only one of them is connected to the unloader valve in this fashion (see the picture - front). The other one uses a solenoid valve on that discharge port. I haven't taken it off to inspect it, but logic would indicate it's got to be a N/O solenoid valve, so it closes when the motor is running and opens when the pressure switch cuts the power to the whole rig, so the valve goes PSSSSSHHHHT, releasing the air in the compressor head via that discharge port of the check valve. Meanwhile, the check valve on the first motor discharges through the thin tube and the unloader valve itself.
Now this is the part that doesn't make sense to me: why would they NOT use a solenoid on the first check valve as well, but instead use the tube leading to the unloader on the pressure switch unit
Having a solenoid seems a bullet-proof solution to ensure no air escapes when the compressor is running (unless the N/O valve itself becomes faulty, which should be easier to replace). I'm pretty sure I can feel a faint puff of air escaping from somewhere around the housing of the pressure switch when the compressor is running.....which may not necessarily mean the unloader is leaking. Could be the pressure switch itself, but having a solenoid on that check valve, instead of that discharge tube leading to the unloader, would eliminate the possibility of that air coming from the compressor through the check valve's discharge port...
In post #11 you mentioned blocking off the unloader port, so I was mostly curious which side you tried plugging up exactly: was it the check valve end, or the the pressure switch end ? From what I understand, the unloader end (in the pressure switch) isn't actually connected to the tank itself, so if you were to plug that side, there should be no pressure there. Only if you tried to plug the side coming from the aluminum tube from the check valve there would be pressure, as it's coming straight from the motor (when it's running).....yet you say had some pressure there. Then again, you said it was hard to get to, so maybe you are referring to the unloader side after all (since on mine, it does seem quite tough to disconnect the tube from the underside of the pressure switch without taking off the entire assembly, gauges, regulator and all), but then where was all that pressure coming from ? From what I gathered, the unloader is simply actuated by the pressure switch - it is not actually connected to any pressure, so if I were to disconnect it, nothing should happen. It would just sit there doing nothing...
The reason I'm interested is because I was planning to install a solenoid on the first check valve as well and just ditch that return tube entirely, but I'm not sure what happens to the unloader side if I just leave that floating. I might end up with air escaping from the tank if it turns out it's under pressure.