I went on the Brimba website and 3dcontentcentral but I do not know which assembly was used. Could you please provide a link to the piston assembly that is being used? I could be more helpful if I see the problem myself.
I downloaded a Brimba piston assembly and all of the parts were fixed (had a (f) next to their name). To solve this issue just right click and pick float. You might then need to create some mates. Also, the mates in the assembly might be restricting the piston from moving. You can right click on a mate and suppress it to see if it is restricting the piston from moving. This is a bit like deleting the mate but SolidWorks keeps track that the mate was created and it is easy to bring back suppressed mates by right clicking and picking unsuppress
I have created a few videos. I hope these help.
http://youtu.be/qHOoRBexm-A
http://youtu.be/f4M5dUT1CBg
http://youtu.be/aruQcgKnMQc
http://youtu.be/D0jLpiMw64M
If this does not solve your problem, could you please tell me what you want the motion study to do with the piston besides just move it. As in what do you want to analyze or what results you want to obtain from the motion study? If the problem is in getting the pistons to move, I would be happy to look at the assembly and make a few suggestions.