68' DART BRAKE WOES

That era car had res line pressure valves to drum brakes & will NOT gravity bleed. Also drum brakes need an initial adjustment to minimise wheel cyl travel. Excessive travel will result in a low pedal.

Pedal goes to floor & is firm. Is it firm because there is no air in the system? If so, it needs a p'rod adjustment/change to reduce travel.
If it is firm because the M/C internals have bottomed out, then there is air in the system, faulty m/c or drum brake adjustment.