So many of these Chinese 2-stage compressors (HPDAVV/HPDMC, Tuxing, Air Venturi, etc.) seem to start with the almost identical motor/compressor and frame components, and then differ significantly on their cooling and control systems.
My (aptly named) Diablo compressors use the same base motor/compressor/frame. Long story short, I ended up purchasing 5 of them cheap off eBay, but they were all such a mess that I attempted to return them to the seller. He refunded my money in total, and said to just keep them. Free compressors give a retired person a lot of incentive to figure out how they work, and how to fix/rebuild them. 4 of the original 5 units now work great (although the one with the longest run time currently has only about 12.5 hours). I have given 2 away free to friends who I have hooked on PCP air guns. I use one almost daily to fill guns directly, and occasionally top off tanks. I have one good 'spare' unit. So I should be happy. But I haven't been able to fix that 5th unit, and your post (and the subsequent replies) caused me to pull it out of the shed and try again tonight.
I am attaching some photos with numbered areas referenced. If your and my cases do relate to check valves, this is what you would likely see if you got into your unit. My 'bad' #5 unit has the following issues: Volume lower than the others, only builds pressure to ~230 bar, and the worst thing is that it blows lots of oil smoke out of the crankcase breather. This last issue is what leads me to believe that I have check-valve issues. I have already torn both sides of this compressor apart, made sure my piston rings were staggered, replaced the first stage valve plate, replaced the high speed rings on the smallest piston shaft, and generally cleaned things up. Performance got better, but still not 'good'. This unit currently has Mobil Rarus 427 oil in the crankcase, but if I get it working better I will be changing over to synthetic SecoLube.
BTW: I HAVE FOUND WITH THESE COMPRESSORS THAT EVEN A SMALL AIR LEAK WILL MAKE THE UNIT SEEM LIKE IT IS NOT BUILDING PRESSURE AT ALL. They don't put out an enormous air volume after all, so even a tiny leak impacts them in a big way. I have kind of seen the gamut of how these compressors can fail to work and early on when my first one wouldn't build pressure it was ultimately due to a small air leak. CRC Aerosol Leak Detector is your best friend as the radiator cooling fan and the motor's in-built cooling fan both blow so much air that a thick and tacky leak detector is needed.
Numbered areas:
1) Outlet of first (low pressure) stage;
2) Inlet of second (high pressure) stage;
3) Outlet of second (high pressure) stage;
4) Inlet to the desiccant filled manifold 'tower'.
In the last photo you can see the various check valves you will likely find in your compressor.
I am going to clean up a bunch of carbon deposits on my components, and see if any of my previously purchased re-build kits (Tuxing, through AliExpress) have suitable replacements. Maybe I can get #5 working after all.
Anyway, I at least wanted to post these pictures to let you know what you would be getting into. Not that bad actually. But do take digital photos as you are taking things apart, and have a nice big surface available with a towel on it to keep small bits from rolling away.
Good luck!
