No cause found for persistent pleural effusions

I have been treated for 2 years for pleural effusions. All fluid and blood work comes back clean - no cancer, infection, virus - nothing. They had been tapping both lungs and getting 1.5 to 2 liters from each side. The heart has been checked out (EKGs, Echos, stress tests) and there are no issues.
In April I had a bi lateral pleurodesis performed with biopsies from my left lung (no issues). They used talc and 7 months later I just had 1.8 liters drawn from the right lung. They believe the left side was moderately successful but the right side failed. I'm about to schedule a redo on the right, perhaps using the chemical option. I am also looking at possible "constrictive pericarditis" and will get another echo today, possibly leading to a heart cath to look closer.
I am a non smoker, relatively fit. I ran marathons and triathlons, ate appropriately, non diabetic. Only medication is for cholesterol management and inhalers to try to relieve my breathing discomfort.
I'd like to treat the cause - not the symptoms.

Any ideas?
Rob C.