I believe that the hydraulic piston should be added to the list of simple machines.
Now, that's a pretty bold statement there. We all know the simple machines - lever, wheel and axle, pulley, inclined plane... the screw is debatable. The backhoe is not. But, I feel, if we look at the rationale behind the list, we see that the hydraulic piston fits with all the others.
Simple machines allow us to alter kinetic force. We can change the direction of it, or trade force for distance, thusly:
The hydraulic piston works because liquid does not compress (that's why you need to pump air into your Super Soaker). As the liquid on one side is pushed down, the liquid on the other side pushes up. You can change the direction of your force, or exchange force for distance by varying piston surface area.
The hydraulic piston, as pictured here, cannot be reduced. It only looks more complex than the other simple machines because the liquid needs to be contained. The hydraulic piston may not have the same historic authority as the lever or pulley, but it does the same thing and is irreducible. It is my firm belief that it should be taught to grade schoolers as one of the simple machines.