Hello guys: One of our CAT 416 had an important hydraulic leak. And it only stopped when the tank was half empty. So we brought the machine and disassemble and for our surprise we didn't find 2 out of 6 bolts I was thinking in buying a new tank but that was it. Changed the o-ring, put the 6 bolts and the leak stopped
The 2 bolts vibrated out maybe ..? Always a good idea to install bolts like that with Loctite or pipe sealer when installing just to prevent the type of leak you had.