I'm working on the brakes on my dad's 91 C1500. I've replaced the booster and master cylinder. The rear cylinders and calipers are fine so are the pads, disks, and drums. The problem is that the brake light comes when I push the pedal to the floor. I've checked the lines and there aren't any leaks. When I bled the rear there wasn't hardly any fluid coming through the bleeder screw and plastic line. Could there be dirt in the line that's blocking the flow? If I disconnected the hard line from the rear cylinders, could i use compressed air (at a low psi) to clear the lines at the junctuion block? Any other suggestions or commenst would be great?