Questions about my brakes

I want to check my brakes on my '66 Barracuda. The level is very low in the MC and the brakes are spongy. What I plan to do is inspect the brakes, and if they are still good, keep them, but I feel that it would be a good idea to flush the system before adding and bleeding through new fluid. Also, what type of fluid should I use, as I don't know what was used in the car before? Thanks.