Brake Problems in 84?

[benscool1]

The problem just started this week, we put a new rear brake line on a few weeks ago because it busted when I started towing another truck so we fixed it in the parking lot of the autozone about a mile from where I bought the truck. Now suddenly whenever I push the pedal down there is nothing for about the first inch, but once I get past that, suddenly the wheels lock up, and sometimes it seems like its just the drivers side rear. I don't really mind it because it doesn't happen all the time, or at least not as bad but I have new tires and I don't wanna pay for new ones! Its not using or losing fluid, and I checked and saw no leaks underneath. What could it be? cylinder? proportioning valve? master cylinder or booster?