Attempted to bleed the brakes on my 67 dart tonight, but couldnt get a firm pedal. They're manual with a dual reservoir and the front disk swap.
No leaks through the system, but i was getting what appeared to be foam/air in the rear reservoir. I had the m/c cap on at all times unless i was refilling it.
Is my cap seal just bad (looked fine with no dry rot or swelling) or is it possible my M/C has an internal leak?

cheak the firewall behind the booster for fluid and check between the M/C and booster for fluid. the best way i found to bleed brakes is with a pump w/ resivoir and pump from the caliper up so the air pushes up threw the M/C. makes it a one person job and ensures no air. once you know your system is airless it will be easier to tell what causing your problem.

You don't state why you needed to bleed the brakes. Did you replace parts or was the pedal just mushy? If it was just a musy pedal then I would say you have a leak thet is very small. This could be a rubber line or as you noted the master cylinder. Need more info.

Are the disc bleeders facing up?Dont ask how I know.Rocky

Pump slower. You need to allow the air to escape.

Gravity bleed them