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I’m sure this has been answered before, based on your introduction that indicates there are thousands of questions and answers but I can’t seem to find it (the search box above searches the WHOLE INTERNET, not just your site, so I end up with computer answers and other non-relevant stuff). So I apologize in advance!

I have an Emglo AirMate Model #AM78 HC4V that I inherited from my Dad, so I am not real familiar with it, nor it’s history. It is marked “125 PSI” on a sticker but it is not shutting off when it reaches that pressure (on what I consider the “Primary” gauge, not the adjustable one at the air supply). It continues to build pressure and then what I call the “Pop-off Valve” blows and it drops pressure immediately and I shut it off manually via the manual ON/OFF switch.

Check Valve Leaking On Emglo 21868172
Check Valve Leaking On Emglo 21868172
Emglo airmate AM780 HC4V parts list

I’ve included a couple of photos – one of the overall compressor, plus one of what I call the “Pressure Switch” and one of the “Pop-off Valve”. In the pressure switch photo – I’ve outlined what I call the “Pressure Switch” in GREEN. I’ve outlined the “Pop-Off Valve” in BLUE and what I assume is some kind of “Sensor Supply Tube” in RED, which appears to supply air pressure up to the switch so that it knows when to shut off.

We were borrowing the compressor last year for my Son’s Eagle Scout project and the compressor was working fine until the “Sensor Supply Tube” split (outlined in RED in the photo), so I replaced that with similar tubing from Home Depot or auto supply store – I don’t remember, but it seems to be OK now…

Then, right away when we started using it again, my current symptom started – it builds pressure in the tank, but never shuts off and then the “Pop-Off Valve” blows at about 140PSI or so.

Just providing the above regarding replacement of the tubing for background, since I don’t know if these two issues are related or not??? Maybe it wasn’t shutting off properly and that’s why the “Sensor Supply Tube” split in the first place (too much pressure before the pop-off valve blew??) or perhaps when the tube split, it has a diaphragm or other mechanism within the pressure switch that was also damaged or needs reset or replaced???

Anyway – I’m assuming that it’s the pressure switch that’s not functioning now, but is there a way to repair?? Or at least test to confirm before purchasing a new one (if they’re even available for this old compressor)??

Thoughts? BTW – you have a very informative site here and I can’t wait to delve into it more to better understand my compressors!!

So says Scott, from Indianapolis, IN.


Scott, regarding your Emglo AirMate Model #AM78 HC4V, the pressure switch tube you refer to  is the unloader valve line. The purpose of this line is to vent air from over the piston when the compressor stops. That it split before, to me, has no bearing on the issue you are facing with this compressor now.

The blue circle is the PRV. This one sure looks like it needs checking to be sure that it’s working as built. However, since it’s cracking at the rated 140 PSI, that’s a good indication that it’s still OK.

Off hand and based on the symptoms you describe so well, I’d say that you have a pressure switch issue.

If the tank pressure bypasses the normal set point for that compressor, and motor continues to run, the PRV lets go at a certain point, then the pressure switch failed to cut pressure at the cut out set point, and to me, it’s past time to replace it.

If this were my  compressor, I’d get the pressure switch fixed or replaced before I used the compressor again. Fixing them is possible, and there are pages on this site linked from the troubleshooting page that tell how to do so, but as I’ve often said, shopping on line for replacement means finding one that’s under $30 typically, and it’s not worth my time and the aggro necessary to rebuild it. I’d rather just get a new one.