'86 Case 580SE Brake chatter

When heading downhill and applying the brake on my 580, I get very loud brake chatter. Kinda sounds like rocks being crushed. I have learned that this is most likely due to old hydraulic fluid which causes the wet brakes to "stick-slip". Is there a additive I can put into the hyd. oil to improve/fix this in order to prevent having to flush out the entire hyd. oil?
Any help is greatly appreciated!

'86 Case 580SE Brake chatter