Final drive is building pressure with hydraulic oil. Duo cone seals on propel motor or o rings in brakes? or both? Can you cap off the brake hose and see if the fluid stops? How hard is it to change either on 690ELC.
Check the case drain line (return to tank) Click link below and see troubleshooting section
Had a hitachi that they brought the final drive to me with the seal in travel motor blown and the seals in final blown inspected motor and replaced seals in both and gave unit back and told them to check case drain line before they cranked back up, didn't happen, walked a few feet and did the same thing, brought unit back and I resealed both again and went and put the unit on myself. Found pieces of rubber just before the T where the right and left case drain lines combine into 1 line before they go into swivel, cleaned and problem gone.