I found this old model of hydraulics that someone had lovingly made. The idea is that when you push one syringe, it puts pressure on the liquid. Liquid is incompressible because the particles in at are basically touching each other. So the pressure transmits through the liquid to the other end. The same pressure acts at the other end and causes a force to act on the plunger at the other end, pushing it out. This is how car brake pedals push on the brake pads that are attached to the wheels.
But the model has seen better days. Air has got into it. When you push on air, the particles are far enough apart to be pushed closer together. The pressure at the far end is not the pressure that you push in. The brake pads would not push on the wheels as hard.
