Fliud around my dogs lungs wont stop accumulating

My 11 year old wire haired pointing griffon seemed to be having a hard time breathing. We took him to the vet and he was there for a week while the vet diagnosed him with pleural effusion and put a chest tube in place to treat it. Tests of the fluid came back completely negative for cancer and infection. Scans show a mass near his heart. We suspect the fluid is coming from the mass but cannot figure out truly why it’s accumulating. He is on antibiotics and steroids to slow the fluid. Steroids have slowed the accumulation but it is still g a steady ~200-300 ml per 24 hours. Any help would be so appreciated as we cannot keep the chest tube in forever): today the vet suggested seeing a specialist($$) or a “pleurodesis” would this really work since we dont knownthe underlying cause?? does anyone have any tips to help save my best buddy?