Composite propane tank leak

I've got 2 composite 10kg/22lb gas tanks which have both expired in 2017. I got them from my father some years ago, he told me they couldn't be recertified because they were discontinued or something. I figured I could use them as air tanks for my pneumatic projects so I purged them, making sure there was no propane left in them.

About 2 years ago I noticed the rings around the necks of the tanks said: country of origin - some numbers - PH 30BAR - 12.0L. So the test pressure of these tanks is 30 bar. For some reason I then decided to pressure test them using a HVAC scroll compressor. I took the entire setup to a safe place and filled them to 30 bar. After leaving the tanks for a while I stored them in a shed, still filled. Some months later while cleaning up the shed I emptied them. I didn't check the pressure of the tanks, but there seemed to be a reasonable amount of air in them.

I haven't used the tanks since, until today. While filling one of the tanks to 4 bar I noticed it was leaking at the neck. I filled the other tank to 7 bar and it seems to be fine. I was kind of surprised as they seemed to have survived 30 bar of pressure and are meant for propane, which of course, shouldn't be leaking out of a tank.

Does anyone know what might've caused the tank to leak?

Factors which may've had influence:
Storing them at 30 bar
Possible temperature changes while storing
Small amount of oil in the tank from the HVAC compressor
 

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Are you aware that ‘playing’ pressure testing is dangerous and that for formal pressure testing there are Statutory Procedures that must be followed by competent persons (in the UK but similar regulations will apply elsewhere).

The 30bar pressure test is for the cylinder construction - what is the maximum allowable pressure of the components above the cylinder neck, because I’m pretty certain it won’t be 30bar. And yet you admit leaving them highly pressurised. Any failure of the above the neck components could have left a shed as fast (and as dangerous) as a bullet from a gun.

What caused a tank to leak - complete lack of maintenance !
Commercially each time a cylinder is filled with whatever product, the integrity of every component/seal are checked.
 
I'm aware of the dangers. There were no components on the closed valve after filling the tanks and all components used were rated for the pressures used. The problem is that there's air leaking through the fibers of the tank, near the neck.
 
If you know anything about pressure vessels then you will know fibre washers are commonly used to seal (screw) joints but have a limited life so are regularly replaced.
 
The problem is not the valve or a washer in a component. The problem is air leaking through the wall of the tank, near the neck. I don't understand how a modern propane tank could have a leak like this.
 
OK when you mean fibers in the tank, you mean tank wall.

Guessing it’s a failed weld or crack in the steel casing - it’s failed the pressure test !
You don’t know how these cylinder have been treated in the past. It’s not unusual to have them dropped or handled Roughly.
 
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