Brake Flush issues

[bugsy]

Decided to do a brake flush on our 70Z that we own for 16 years. I removed the old fluid out of the master cylinder which was dark in color. I hooked the Matco mighty vac up to r/r bleeder valve. Hardly anything came out of both left and right rear bleeder valves. Front ones I got fluid out. So now I try and manual bleed them. Put my wife in the car, and I got some fluid out of both rear bleeder valves. Tried it again I got nothing. Seems like brakes apply , but not all the time. Fronts are good. This is a 33k survivor car and I am trying to figure out if its the wheel cylinders. Anybody have these issues. Bleeder valves are not clogged. Still has original blue caps on front bleeder valves. Any help would be greatly appreciated. Thank you.