This is what happens when the elevator gets low on oil: The hydraulic elevator uses a pump to force hydraulic oil into a cylinder, thus pushing a piston upward. The elevator car is attached to this piston and therefore moves with the piston. When the pump pushes oil into the cylinder, the elevator moves up. When a valve releases this oil back into the oil tank, the elevator lowers. As the elevator ascends, the oil level in the tank lowers; as the elevator descends, the oil is pushed from the cylinder back into the tank, thus refilling it. There is a little more than enough oil in the tank to move the elevator to the top floor, however, a small amount of oil can sometimes leak out of the system. If enough oil leaks out, the pump will not be fully submerged in oil when the elevator is reaching the top floor, so it will suck in a mixture of air and hydraulic oil. This can cause the elevator to bounce slightly when it is leveling with the top floor.
This is the same as drinking water from a straw, with the exception that you are not putting the water back in the glass after you drink it. When the straw is fully submerged in the water, you do not suck any air. When the straw is not fully submerged in water, you suck in a mixture of air and water, creating the “slurping” effect.