DGA1
I'm Awesome
If you have fluid in the booster, replace the MC, like you said seals are bad. If the MC went dry, you might have to bench bleed it again. Sure it's more hassle, but only way to ensure air is not trapped in there. While you have it out, take it down to your local parts store with a $20 bill and exchange it for a new one. 
The MC could be bad internally too. Shot seals on the piston will make it cross bleed internally giving you a sinking pedal. Either way, I'd start at the MC, either a new one, or bench bleed your current one. Bleed the whole system. Adjust the rear drums where they barely drag. Remove the ABS fuse.
This is what I did with mine. Stainless steel lines, all 3. 1-ton front calipers. JB7 master cylinder. Since one of my axle seals was leaking, new rear shoes, hardware and cylinders. Pulled the ABS fuse. Even with over 4,000 lbs of dirt in the truck, it stops with authority.
The MC could be bad internally too. Shot seals on the piston will make it cross bleed internally giving you a sinking pedal. Either way, I'd start at the MC, either a new one, or bench bleed your current one. Bleed the whole system. Adjust the rear drums where they barely drag. Remove the ABS fuse.
This is what I did with mine. Stainless steel lines, all 3. 1-ton front calipers. JB7 master cylinder. Since one of my axle seals was leaking, new rear shoes, hardware and cylinders. Pulled the ABS fuse. Even with over 4,000 lbs of dirt in the truck, it stops with authority.
