Hi!

It's probably the tank that needs replacement! 18 years is a lot! Usually the black rubber expander in the front of the tank gets hard and leak.
Replace it or replace the tank.
Best tank, used by pylon racers all over the world is the Tettra "Bubbleless" tank. If you choose the Tettra tank you don´t need to worry about any rubber expander leaking because there is none!