Scored On An Air Compressor!

One thing to check and you may have already. Where the large copper tube goes into the tank, that fitting is a check valve. If you have not, pull it out and check that the disc is intact and there is not any dirt ect that will keep it from sealing. If it doesn't seal what you will notice is that after the tank pumps full it will keep pressure on the pump head and it will not want to restart until almost all the pressure is bled out of the tank. You would also notice that after it initially shuts off the air continues to bleed off more than the first couple of seconds.