Yong Heng high pressure (4500 psi) air compressor venting out of top instead of filli

[Jambi1369]

Hello. I have a yong heng high pressure air compressor that I use for filling my HPA air tanks that I use on my markers. I got it on Amazon. Yesterday was the first time I ran this thing. Here’s my issue :

I have a few 90 ci (cubic inch) 4500 psi HPA air tanks and a few 80 ci 4500 psi HPA air tanks. Yesterday I filled 2 of my 90 ci 4500 psi tanks and 1 of my 80 ci 4500 psi tanks. When I ran the compressor it filled all 3 of my 4500 psi tanks to 4500 initially but as they cooled down they all went down about 500 psi each so each one had 4000 psi. This is normal and what happens when the tanks cool down, so I just top them back off after they cool down.

now, when I went to top them off to 4500 I couldn’t get the dials on any of the 3 tanks’ gauges to move at all past 4000 psi. I also started to notice a leak sound along with the hum of the compressor running and I noticed it was coming from around/underneath the nut on top of the central cylinder head (pic included). So I guess that even though when I held the bottles in my hand they were vibrating (feeling like air was indeed going into them) the air was instead venting out from this leaky nut, hence the gauge needles not moving anymore.
Now, I’m thinking 1 of 2 things must be the problem:

first, I’ve heard in reviews that this compressor sometimes has trouble actually filling all the way to 4500 psi even though it’s rated that high. So, I’m thinking, maybe once the tanks each reached 4000 psi in pressure the air compressor couldn’t actually push any more air into them against that much pressure and instead started venting the air out from around that nut instead.

or, second option, the air compressor somehow broke/became damaged and I either have to repair or replace it.

so, sorry about that lengthy post. However, does anyone know which of those options is the culprit (or whatever else is the problem maybe)? And if it is broken/damaged, is anyone able to tell me how I could fix/repair it and what exactly I would need to do this?
Again, sorry for the lengthy post and thank you very much in advance for your help and your time. It is much appreciated. 
- Kris
Here are the pics. First pic is the air compressor I have. Second pic is the zoomed in view of the central cylinder from the first pic and I have circled in blue where the leak is coming from. Thanks again in advance.






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