Classic Mustang leaks tend to be written off as one of the warts of old car ownership, but old cars don't have to be leakers. They leak mostly because we don't take the time to find the cause of the leak, which typically occur when seals and gaskets deteriorate. However, when new seals and gaskets leak shortly after replacement, it means we've missed something important.
Seals and gaskets leak mostly due to poor installation technique. This is why you must tackle seal and gasket replacement methodically for best results. When you remove old seals and gaskets, examine the contact surfaces for irregularities and scoring. Examine seals and gaskets themselves for flaws and improper installation.
Here, we're looking at axle seals and gaskets. Your first clue of a problem is damp brake backing plates and tire sidewalls, which means the shoes and drums are also damp with either brake fluid or gear lube. Gear lube will be found below the axle shaft. Brake fluid will be found below the wheel cylinder.