Adding Brake Fluid to Original System ?

[54blackhornet]

I need to add brake fluid to a original 71 C20. It has power disc/ drum setup and is all original. The master cylinder has never been low and never had problems in over 10 years of owning the truck. Question, the fluid is dark and I figure it's time to flush it out. Good idea or just top off ? What type of fluid to I use ? Flush it ? Thanks it's hot and I got a big fire close by the ranch to watch out for too...