Okay, I’ll try and compress (compress….ha-ha) the tech weenie lecture to the essentials.
Mog airflow:
1) Air filter to compressor intake port
2) Compressor output port into long, fairly large dia. metal tube entering
3) Pressure regulator/ compressor unloader valve/ return check valve feeding
[3A. Air dryer/ water separator, if so equipped, which feeds: ]
4) a. Smaller pipe going forward through check valve to glad hand on front bumper (can be used to fill truck air tanks)
b. 4 circuit protection valve, which uses dual feed lines to the two main reservoirs; other outputs to various locations.
5) Main air tanks provide feedback loop to the press reg (3) to open unloader when max fill (18 bar) is reached, dumping compressor output to atmosphere, and close unloader when either tank falls below 10 bar, sending compressor air output into the truck system (3A-4)
6) From here on, things get messy, complicated and likely rather variable, depending on the truck and equipment.
All constant-run compressors MUST have an unloader. If one contained the output, there is a real limit to ultimate pressure that can be achieved by any compressor, based on physical characteristics, like compression ratio (surprise) and valving. Most piston-type compressors use reed valves, in which down stroke sucks the intake open, and pulls the exhaust valve closed, and vice versa. The air in front of the piston cannot achieve a pressure higher than is created by the compression ratio, because if there were to be higher pressure downstream, the piston could not force the exhaust valve open to push more air downstream. In practical terms, every compressor has a “stall point”, in which only damaging heat can be created, but no additional air storage. One can disconnect power before that point is reached, or one can dump the output; with no back pressure, the compressor free-wheels with minimal heat generation; the output air needs very little pressure to push open the exhaust valve, and there is no restraint in the line, so there can be almost no pressure build-up. The Mog dumps the output to atmospheric, through the pressure regulator/ unloader valve (and the aux port that I use for inflation has no air pressure unless the unloader is closed for the 10-18 bar fill range).
My assertion stands: if you cannot go above 5.5 bar, either the compressor is failed, or there is a way that large amounts of air are being bled off unintentionally. My reasoning for checking the unloader first is that if it is indeed partly open, pressure will not likely get high enough to generate a really load hiss. I would do the remote fill of the twin tanks as the easiest first “section” (4, 5, 6) of the system to evaluate. This should also quickly tell you if the check valve function of the pressure reg is holding. You should be looking for the first device on the feed line from the compressor. It may look like just a pressure regulator, but it is an active device, and it opens, as an unloader, to limit max press. I amp quite sure the first check valve in the air delivery is also part of this device.
With the engine running, you should not hear any venting to atmospheric at the press reg/ unloader (3), assuming you start at less than 10 bar and haven’t risen to 18 bar max. There might be a very brief “dump” at start, but it can’t sustain that dump, unless there is a failed part. There is also likely to be an audible sound of air flowing within the piping.
If you are able to pump up to the 5.5 bar, that should give enough pressure for the soap test on the air line and fittings from the compressor (2)
At that point, you are left with the compressor itself (1). Of course, you can do the tests in reverse order, but I advocate a rigorous methodology, regardless of starting point. One thought: have you ever had the truck running with the compressor air intake hose NOT coming from after the air filter? Meaning, could you have admitted unfiltered air, and gotten a bit of grit inside, messing with a reed valve?
FWIW, I have scanned and attached some pages from my (OM 366A) workshop manual. Might help with diagnostics.