'75 Duster intermittent brake problem

[DSG Duster]

. 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.

 

[TrailBeast]

Master cylinder is going.
When it's cold out the rubber cups are hard and they don't flare out and seal.

There is a diagnostic I use for this.
With a very light pressure slowly push the pedal down and it will probably go all the way to the floor, and sitting in traffic if you let up on the pressure a little it sometimes will also go to the floor. (Especially cold)

 

[player1up]

I had a similar issue, turns out the rear brake shoes weren't adjusted tight enough.
Bled the brakes 20+ times, never fixed it. I ended up swapping to discs in the rear instead of replacing all of the adjusting hardware.
What I did to verify this was to clamp the rear rubber line with vice grips ( protected with cardboard ). Hard pedal instantly with the rear line blocked off.
Still could be your front brakes, but if the bleeders are stuck, time for new calipers.

TrailBeast's diagnostic is spot on if the master cyl is on it's way out. Light pressure on the pedal wont flare the plungers to build pressure, but a quick jab on the pedal will.
I also had this happen with a "rebuilt" one from a parts store. Bench bleed didn't feel quite right, thought I was doing something wrong, but it was bypassing internally right out of the box.

 

[TrailBeast]

I had a similar issue, turns out the rear brake shoes weren't adjusted tight enough.

Setting the E-brake and then feeling the pedal is the way I check the rear adjustment.
The E-brake holds the rear shoes out to the drum so they don't have to travel when you push the brake.

If there is a big difference with the E-brake on or off, it needs adjusted.