Yong Heng Help

If you have no other leaks you may want to check the high pressure check valve. It is on the high pressure outlet block under the fitting that the stainless line is connected to. It is simply a spring with a small piece of brass and a replaceable o-ring. When the o-ring wears or gets old and brittle the check valve will no longer function so the unit cannot build much in the way of air pressure. It's an easy fix.

sambo cotton has the repair at 1:44



https://www.youtube.com/watch?v=YjsBcKSVeGk






 
Ok, the o-ring was bad, so I replaced. Still not working. I did notice something, that little circle at the top, right over piston. I did not along the holes, could that be the issue?

That O ring is just to prevent compressed air flowing back to the compressor. The compressor will still work without that O ring because the reed valve on the top of the 2nd stage cylinder will do the job.
 
Yes, not filling up the scba tank. I sprayed soapy water on it and not seeing leak.

My Yong Heng has failed to build up pressure for a number of times and ALL were found to be caused by leakage. Soapy water can only detect relatively small leaks. If the leak is big, the water will simply be blown away giving no chance for bubbles to be formed. I would suggest you to turn off the compressor and listen for leaks first. You will need to do it in a quiet place.
 
.....tighten the black knob more and it should stop leaking.

You probably have over tightened it resulting in the "needle" going through the hole of the delrin seal. I have made such mistake before. You may consider replacing that seal with spare that came with the Yong Heng. Normally the bleed valve does not require much force to get it closed.