KALEITE compressor issue

Hello , I have a KALEITE compressor ( chinese made ) , and it ran perfectly for 5 years now , filling a 7 liters bootle up to 300 bars easily . But now when it reaches 260 bars , the pressure doesn't increase anymore . the compressor runs normally but the pressure is stucked at 260 bars. I checked for leaks but there is none. Any idea about the problem ? Piston rings worn ? Below is a pic of the machine. Thanks
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By the way , did anybody disassemble the high pressure cylinder ( the one on the right on the picture above ) ? I found a chinese video dealing with that , but there is no explanation ; the piston seems to have grooves on it , but nos rings . That sounds srange to me , but the image is not very clear . The mechanic applied a lot of oil on the piston before installing the cylinder , sealing purpose? I've never disassembled a compressor so I hesitate to do so , but I have to fix the problem of the compressor which doesn't go higher than 260 bars when still running . Video link below.
 
By the way , did anybody disassemble the high pressure cylinder ( the one on the right on the picture above ) ? I found a chinese video dealing with that , but there is no explanation ; the piston seems to have grooves on it , but nos rings . That sounds srange to me , but the image is not very clear . The mechanic applied a lot of oil on the piston before installing the cylinder , sealing purpose? I've never disassembled a compressor so I hesitate to do so , but I have to fix the problem of the compressor which doesn't go higher than 260 bars when still running . Video link below.
I thought the same thing the first time I noticed it. Nonetheless, Bauer, Coltri, and Alkin final stage pistons are similar in design with lubricating grooves but no rings. Evidently they can hold pressure without piston rings.
 
I disassembled all the parts of my Carette compressor , clean everything . I runs a bit better ,until 250 bars . But I noticed something I don't understand . When I connected it to my 7 liters tank, the pressure doen't go higher than 260 bars , even if I let the compressor running for more than an hour. I takes normally 30 minutes to fill it up. But when I close the valve of the tank , or if I put a plug in the line coming from the compressor , it reaches 300 bars easily. So when the compressor runs in closed circuit , it reaches the required pressure.
Here is my question ; if the compressor is running too long , or if there is a problem with the cooling , making the compressor overheating , could be this the reason it can't reach the 300 bars , and stay at +/- 260 bars ??

Thanks for your suggestions.
 
If you close the tank valve and pressure hits 300 bar, it's not the compressor. It's the tank valve.
Yes , that make sense ; i'll try to charge another tank (y)
You either have an undetected leak somewhere or a highly inaccurate gauge or a leak in the tank valve. I sold a Carette and the next owner had the same problem. After he cleaned the second stage check valve assembly above the piston all was back in order.
I checked every possible leak with soapy water , no bubbles , no hissing. I'm going to re-clean the check valve assembly , and check the water pump, 'cause the cylinders seems hotter than usual .
Thanks for your advices.(y)
 
Still trying to restore the 300 bars of pressure on my Carette compressor . When I start the compressor until 200 bars , then switch off , there is some air leaking through the intake air filter , on top of the low pressure cylinder. No leaks on the pressure lines or anywhere else , so I suspect the check valve at the bottom of the air/oil separator , or the check valve on top of the high pressure cylinder.
I just try to understand what's the problem.