1981 Firebird brakes

I have an 81 Firebird that has been parked since 1996. Before it was parked, the brakes worked great, just touching them they would react. I brought this car out again in 2008 and some work needed to be done from sitting so long and the only thing I can't figure out is the spongy brakes. Before parked the calipers and wheel cylinders were replaced along with return springs and just last year a brand new master brake cylinder.

Still spongy brakes even though the brakes have been bled over and over and the rear shoes adjusted tight. One thing a friend of mine had noticed is the sliders on the calipers looked very dirty. Would this be the cause of spongy brakes?

The sliders would make to brakes ineffective and prevent the caliper from moving, I would focus on spongy flex hoses, master cyl. bypassing, rusty rotors and drums.