Fluid Buildup in Eye Socket?????

Was wondering if someone could help. Have a 2 yr old male cat who lost an eye in a battle with some kind of animal. Eyeball is gone. Since we adopted him from a no kill shelter he continues to have clear fluid buildup in the eye socket. Every 5-7weeks we have to take him to the vet to get it drained. One vet tried injecting some kind of antibiotic into the area twice to get it to stop, with no luck. We have been told by what it seems like everyone that it shouldn't happen. Does anyone have any suggestions on treatment, other than going in again and trying to find out what is causing it??
Thanks so much!!!!

The only thing I have ever heard that may cause this, is if they left part of the eye when they did the removal. I only know this because I have a kitty with one eye, and when the vet was doing the surgery I had asked her what they complications may be.