The piston seal is a tan part in the middle of the piston. The wear bands here are black and there are 2 on top and three on the bottom of this particular piston seal. In the 2 pics below, the tan piston seal is puckered and if you look close you see that it is recessed in the groove. This would indicate that the excitor behind it (a soft rubber part) is missing and the teflon seal is just floating in the groove and as such, bypassing fluid. The black debris is crumbled pieces of the excitor.
This piston below has a broken piston seal. Note that the tan part has fallen down around the head and is broken. The excitor is still in place in this pic.
