your statement is not too clear, but let me see if i can give you some direction. if i remember right, you have an 'o2. with one of that age, the petcock could be leaking around the O-rings and it would be a simple fix to replace them. or you could just replace the entire petcock. but going a little deeper, if you find the tank itself leaking from a possible seam, i would take a look at the lining inside the tank. over the years, it may have deteriorated and allowed the gas to get to a seam that was not as strong as it should have been. if this is the case, you would need to have the tank/tanks stripped inside and re-lined. (if you are adventurous, this can be done by you)
it could also be leaking around the crossover pipe that runs between the two tanks. not familiar with how this is set up but someone else will probably chime in on this matter. (my K9 only has a single tank)