When a piston seal goes bad the cylinder acts like a ram. The best way to explain this is that instead of the fluid pushing the piston it fills the entire housing and squeezes the rod out of the cylinder. This makes the cylinder very week in the extend mode and powerless in the retract mode. You will also see the cylinder extract much faster than it normally would.
Remove the cylinder and take it to a cylinder repair shop. Tell them that you want them to only tear it down and inspect it. If it requires more than a simple seal kit replacement I would just purchase a replacement of the cylinder. Any other shop work will likely cost more that the new cylinder.