sounds like a fluid displacement cylinder.
They are commonly used in hydraulic systems on lifttrucks today.

They have a safety valve in the bottom of the piston located in center.
These valves can cause all sort of problem if they fail or become faulty. I have even seen the retaining ring come out and the valve fall out and be destroyed by the pistion.

Make sure it is present, if it is not then you will need to check bottom of cylinder for debris and remove it, then install a new safety valve.

The holes you see in the side of the piston are ports to allow fluid to flow 'a certain way' through the piston and safety valve.