Yes you can do it. Quite easily Hopefully the hydraulic parts have unique names. The solution is simple but takes a bit of fiddling because you have to get the joint rotation angles right. I've done it with my terminator and powerloader models so I know it works. Parent each piston part to the relative arm part that it is a child of. Set the rotation (green/red crosses) so they are central to the rotation centres. Green for rotation control red for rotation end. This is important. Also the xyz axis needs to match that of the arm so when the arm moves the piston moves with it. Once you've done that and the hyrdaulic ends are aligned. You need to "point at". Select upper piston part "point at" lower part. Select lower part and "point at" upper part. Problem solved. If you need more information and a couple examples how this works. email me at Questor@shalako.demon.co.uk and we'll talk about it. You can do this with props or by having the hydraulics as part of the main figure provided they have unique names.