Just for fun, set the emergency brake then retest the foot brake. If the pedal firms up then your rear brake adjustment need tightening up.
It's also possible the calipers are mismounted. Some calipers do not take well to being swapped side for side to mount them on the other side of the spindle. Found that out on my 65 Cuda when I added discs.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
