Here's the current state of rigging:

Anyone know how to fix the problem at frame 16?  When the wheels rotate to exactly 180 degrees the PistonB moves off its rocker.

Also, why doesn't setting Forcelimits on x and y translation of the Piston keep it from wobbling?  It ought to move in a straight line along its rails; limits are on, but it is more influenced by the PistonB (its parent).  How do I keep it in line?