Bicycle rear brake problems?

Hey guys, I would like to begin this by stating that I don't know bicycle terms very well, so if I'm using the wrong terms, please bare with me. Okay so I have a crappy mongoose mountain bike. Me and my friends found a trail to go riding at. The trail has alot of twists and turns, and most of the path is covered with pine-straw, which brings along problems for me. Because I didn't have back brakes, I couldn't take sharp turns very fast. I was having to use front brakes, which was very dificult because on pine-straw, the front wheel doesn't even have to lock up to lose traction. Thats why I need my back brake. The brake hasn't been working for a few months now, but it was never really that necessary up until about now. I don't want to take it to a bike shop. I believe the problem is that the brake line doesn't have enough tension, so I tried to fix that. On the rear brake, I pulled the brake line very tight, but that caused both of the brake pads to press up against the wheels. So I loostened them a little bit, and I lightly pulled the brake lever, and it still had a good bit of tension, and the brake pads were not touching. So I took it for a little stroll, but after one or two times of about 50% braking, the line lost a little bit of tension. It was certainly better than before, but not enough to really help me on the trail. So I tightened a screw that was on the rear brake lever, and it pushed the lever outward, creating more tension. Again it still wasn't enough tension. Is there anything else I can do? The screw on the handle bars is as tight as I can get it, and I pulled the line out at the rear brake as much as possible with out the pads touching the rim. What do I do now?