Espresso Machine Part I: Pumpless Espresso Makers
The whole purpose of an espresso machine is to force hot water through the coffee grounds. Basic espresso machines (those without pumps) utilize the pressure that comes from heating water inside a sealed vessel.
The coffee grounds are placed into a funnel-shaped piece of metal with a tube that is connected to the bottom of the reservoir. The reservoir is filled with few ounces of water and a top is screwed on.
The water is then heated, causing pressure to build up inside the vessel. Because the end of the tube is under water, the pressure forces the hot water up through the tube. The only place for the water to go is up the tube, through the coffee and out of the tube in the top.
The major problem with espresso makers like this that the temperature needed to build up enough pressure to force the water through the coffee might end up hotter than you want it for espresso brewing.