'75 Duster intermittent brake problem

. My '75 has manual brakes, front discs. When I got it, it had a trailer braking system plumbed into the rear brakes at the master cylinder. I removed it a while ago, but recently have had a problem with the pedal soft at times, & sometimes going to the floor. I bled the rears, & planned to bleed the fronts also, but the bleeders are completely seized. Problem is still there, and seems to exist more so first thing in the morning, when it's cold outside. When the pedal softens, you can pump the pedal a couple of times, & it comes up firm, at which time you can jam it on, and everything works as it should. Front brakes, or master cylinder? Rears are all good. The intermittent failure is what bugs me. Any thoughts?
Thanks in advance, Dale.