How to repair brakes on New Holland TL80?

The brake fluid in my TL80 tractor keeps leaking out somewhere internally in my tractor. Inside the brake reservoir, I can feel a small gasket/filter type item. I can move it back to where I believe it belongs. I add brake/hydraulic fluid (expensive) and I'll have brakes for week or so and then I have to repeat this entire process. Does anyone know how difficult it would be to repair this problem?