control arm for JD 2155

On the first day of my new golf course job I was asked to change the bushings and seals for the control arm shaft. I took the lift arms off and removed the control arm shaft but there is a small piston that rides on the shaft and is now in the way and I can not reincert the shaft. The pistion is under pressure. I can move it, but it comes right back out in the way. WHat is this pistion and how do I move it out of the way? I guess it is hydrolic? I don't know.

Radar:homer: