87 D150 intermittent brake problem

Just recently bought this beater truck. With all the various parts I've bought, I'm approaching what I paid for the truck! In any case here's my problem.
Replaced one of the rear brake hard lines due to a leak. Bled the rear lines. All seemed ok for a couple short trips. Then pedal suddenly went almost to the floor with little stopping power. Thought I had another leak or air in the system. Bled the rear again, attempted to bleed the front...bleeder valves rusted shut, so I replaced the calipers (good thing too since the pads were pretty bad). Bled all four wheels and test drove truck. Brakes worked GREAT!
Next couple trips all well. Latest trip the brake pedal would work great with little travel same as the previous trips after new the calipers, but occasionally would travel nearly to the floor with poor stopping power. It seemed to do this more often if I would lightly apply the brakes, then let up on the pedel, then reapply. Does this sound like there is still air in the system or an intermittently failing master cylinder? Being an intermittent problem has me wondering since it doesn't seem like a "normal" problem.
Thoughts?
Thanks, Eric
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Replace the master cylinder, it's bypassing on you.
And be quick about it before you wreck the truck.
--
Budd Cochran

John 3:16-17, Ephesians 2:8-9, Romans 3:23, 6:23
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It's the master cylinder. I hit a Ford Explorer with my '68 Fury last year, same deal. Really tee'd me off it did. No harm to the Explorer and a caved front bumper on the Fury. No big deal, off with the old on with the undamaged one.
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