Yong Heng Pressure on gauge is not rising.

Start with looking for leaks, why are you letting it get to 75c? That's obviously way too hot, you most likely have a leak somewhere causing the pump to work too hard and make HEAT. If your burst disc went bad I doubt it would make any pressure, just a guess.

I haven't heard you mention cooling water either, just go around with it running and find where the air is leaking from. Hard lines to the head etc, connections, the pressure relief valve bronze looking thing with the ring on it. I can fill to 4500 with a maximum temp of 50c or so.

It’s either the burst disc (not loud, just a minor r hissing noise). I would check that first. Next I’d ensure the check valve on first stage outlet wasn’t leaking. After that you t could be the HP inlet or outlet check valves. Then HP pencil piston rings. Also, like Glem said, you let it get way too hot. 65C operational limit and shut it off at 70 C. 

Sounds like maybe you just got your compressor - is that correct? I had a leak in one of the fittings wasn't properly tightened. Check all your fittings and make sure they are tight. Actually, the leaking one in mine was the fitting that held the burst disc - mine initially wouldn't go higher than 3900 psi but once I tightened the fitting for the burst disc, it rose without trouble to 4900 psi (where I shut it off). I agree that the first thing to check is your burst disc. Take a thin strip of tissue paper and hold it so that it hangs down in front of the burst disc - if it has burst you should see it blowing air on the tissue. The compressor is so loud that there's no way to hear any hissing. Or like you said, spray some soapy water around on all the fittings and look for bubbles.

Even when mine ran for 8 minutes just venting to atmosphere, I never reached anywhere close to 75C. I suspect that you have alot of carbon buildup on your 2nd stage piston. What oil did you put into it? I suggest using a high-temperature hydraulic fluid - Centercut uses Royal Purple and that's maybe the best choice of readily available oils. I use Mag 1 because it's what I had in the lab. At first mine wouldn't turn over at all - I pulled off the 2nd stage cylinder and found alot (I mean "globs") of carbon deposit on the piston and semi-metallic rings. Once I wiped it off (and chg'd the oil), it ran fine and the water temperature now stays under 60C.

I've seen the 2nd stage steel tube (the skinny one coming out of the top of the compressor) reach temperatures of almost 150C - that means the temperature at the interface between the sliding piston seals and the cylinder is likely to be much higher even if just briefly during the compression cycle. Most hydraulic fluids have a flash points under 250C so I could believe that many hydraulic oils do in fact diesel slightly in the 2nd stage and create the carbon buildup (Mag 1 flashes at 235C and I think even though it's a great hydraulic fluid, that's causing the gradual buildup of carbon on the 2nd stage piston). I'm actually running experiments on this right now - but it's a bit of a pain because I have to stop pumping every 10 minutes and disassemble the 2nd stage to inspect for carbon buildup then put everything back together and run another 10 minutes, etc.

It’s sandwiched between the fitting you removed. Prfssrlee has a better more detailed explanation above. But I would recommend the Royal Purple Synthetic Compressor oil. Also, sometimes the check valves stick or gum up and don’t seal completely. Each cylinder has one on inlet and outlet. It’s very simple looking, small disc with spring. 

Oh, check the tightness of the fittings on the vent valves as well. Mine arrived with one of those very loose - just finger tight.

If you are dieseling your hydraulic fluid, then another good possibility is that some carbon has gotten into one of the check valves and is preventing it from sealing cleanly, as Centercut suggests. Pull it out and wipe it clean. For grins, tomorrow just take off the tube coming out of the top of the 2nd stage and look down there - you'll be looking at the top of a flap valve - see if you have any carbon deposit on it. On mine when I first took that fitting off, the carbon buildup had even worked its way up into the threads. If you've burned any oil, you can smell it too when you remove that fitting.

after using mine for only about 4 hours, the gauge took a dump....before it failed, it was actin all goofy. got a air ventury gauge at pyramid air. it has the m10-1.0 threads it needs. all is good.

lkwan78 did you ever find air leak?

Broken flap valve in the 2-nd stage or build up of carbon in the two flap valves in the head of 2-nd stage is the issue ... Now Imsearching These flap valves , but Im not able to find it :-(

In the other thread that you posted this, I put a link to an Aliexpress store that was willing to pull off some specific parts to a YH and sell them. I sent a picture with redlines on it, and they just posted that picture, untouched, and sold me the parts. Here is another store I found that looks like they might do the same, as they have several single pieces and parts for YH compressors for sale that I haven't seen elsewhere (like the capacitor for the motor, the front cover for the crankcase, the oil breather, etc). You also might try them as it looks like they part out YH compressors for sale. The store name is Lj High Pressure Co. Here's the aliexpress link: https://www.aliexpress.com/store/2305020?spm=2114.10010108.100005.2.364d2be42xrri2

Again, good luck!

Again many thanks, your the best

Hi lkwan78, This one is pretty extensive: The 1st stage is the black cylinder with heat fins on it. The air exits out the steel tube coming out the side of the aluminum disc sitting on top of the black cylinder. It goes in the top of the smaller diameter aluminum cylinder, which is the 2nd stage. Also, the gasket between the 1st and 2nd stage is fairly robust - if you want to inspect your 2nd stage piston, just take off the steel tubes then remove the 4 socket head screws that hold the 2nd stage cylinder down to the 1st stage cylinder and life straight up to expose the 2nd stage piston and its semi-metallic rings. But that won't be the cause of an air leak.

To find leaks the best is to mix up some dish soap and water, pour into a spray bottle and spray the pump and lines while its running. Look for sudds and bubbles,that will indicate leaking parts. 

Did you fix this? Mine was doing that. It was the first stage exit port spring.
1/4x1/2 little spring that prevents backflow mine came with a spare. Can send pics if needed.