Fuel vapors...

[RUN666]

For information: I disassembled and reassembled the black cap (the one that locks) twice to check if the valve (brown rubber) worked well, it was ok allowing the tank to suck in outside air. In my problem I had pressure that could not escape anywhere other than through the cap so I was getting steam, or even gasoline, on the tank each time I opened it!
I hope that my intervention earlier resolved the problem which could also come from one of the valves which would be defective (I did not check them except number 38).