When I use slime blocks to attach a water or lava vessel (a liquid source block surrounded on all four sides and below by solid blocks) to a piston and then move it, the liquid disappears or moves relative to the vessel. (If the piston is horizontal, the fluid is water and the block under it is not slime, it jumps to the block behind the piston. If the piston is pushing downward, the fluid does not move and begins spilling over the lowered sides. In all other cases, the fluid disappears.)