Getting frustrated with brakes.....

[FLYNAVY30]

My brakes have felt like crap since I bought my 72 C20 back in May. At first, I replaced all 4 bleeder screws since they were all clogged with rust. Then I bled the entire system with 3 full bottles of fresh brake fluid with a Motive power bleeder. That helped a little bit, but they still didnt feel like they should so I replaced the master cylinder, booster, and both front calipers and pads. I bought new drums, shoes, and cylinders for the rears, but havent gotten to them yet. I then bled the system thoroughly again, and it was good for a couple of weeks. I went out to take the truck over to Home Depot this morning, and it feels like mush again. The pedal has to be all the way to the limit of its downward travel to get any stopping force, and thats minimal at best.

Anyone have any ideas? Im completely lost. Its not a complicated system. The only thing I can think of is that Im somehow sucking air into the system, but if that was happening, there would be a leak someplace, and as far as I can tell, I dont have any leaks....the master cylinder is still full......