johndeere 300b backhoe problems

I have a john deer 300 backhoe that is blowing the seal and even broken the bolt off that holds the canister cover over the hydraulic filter. Is this being caused by a clog in the hydraulic lines or a check valve stuck. If so where are they located? I was told it could be a relief valve problem at the hydraulic pump at the front of the backhoe . I really don't know where to start to solve the issue . Any experienced advice would be greatly appreciated. thank you