Government employee - went on long leave - claim pension

Hi , my dad worked as a central government employee for 25 years and then he got a job at Singapore , he left to Singapore for almost 15 yeara and then came back to India. While he left he went on a long leave and the govt office also did not relieve him or an exit letter was provided after many years gone. Now he is seeking pension for the years he worked there. But he is having a tough time claiming this . They are not helping much. What are the best ways to seek the pension and legal procedures involved? Please help