The fuel lines leak? Or do the fittings on the end leak? Fuel lines that are flexible on a Diesel are usually the supply lines from the tank to the fuel injection unit.
There is nothing inherently wrong with flexible lines used on your tractor. Solid metal lines are a real hassle to install and may not be the best solution. Sometimes you need the flexible lines as a vibration isolation. For instance between the motor and the frame mounted fuel tank. Braided lines look flashy but offer little more as a replacement for a plain old stock black hose low pressure line. I've found that unless you can get a pre-made, fittings attached, exact fit then braided hose it isn't worth the fabrication hassle to put a braided line in.
You need to post more information, or pictures, to really get a good diagnosis. But, here are a couple of stabs in the dark.
The fuel line is actually leaking. You could have a wear point in the flexible line where the line was vibrating against some solid metal. Neither a braided line or solid line will fix this, you'll need to replace the line and re-route the line and tie-wrap it down so it doesn't vibrate against any fixed objects.
You may have a fitting leak and the fuel is running down the hose and dripping from the low point on the line. Closely inspect the end fittings and see if they are loose. You'll find a slow leak using clean rags to check and then re-check the fittings. Take a dry rag/paper towel and wipe the line and fittings. Come back and do the same a day later. The damp spots are the source of the leak. If you get fuel wipe off (a damp spot) and it is a high point on the line, you may have a leaking joint. In any case, check both the fitting attachment to the hose and the fitting to the fixture. If the attachment to the hose is a crimp fitting and appears bad you'll have to replace the line.
Don't rule out that the tank is leaking and then flowing down onto the fuel line (over all the fittings). On old motorcycles this usually the cause for fuel leaks.