How do espresso machines hold pressure at all?
I can't speak for the new high end machines that provide pressure profiling, or the cheap domestic machines, which work who knows how, but the standard system for distributing pressure on the coffee is the e61 group head, and most machine group heads are based off the original design.



The e61 group is pretty much just a system of 3 pressure valves that allows the pressure to slowly ramp up 9 bars and then maintains that 9 bars of pressure while the pump continues to work. Some versions include other steps such as pre-infusion, or on newer high-end machines the ability to create pressure profiles which adjust the pressure at any point during the espresso shot. 

Also note that all the resistance HAS to come from the coffee puck, so you may be grinding too coarse or not tamping hard enough. If you need to measure the pressure from the grouphead, espresso machine techs use a device like this.



You could probably make your own at home if you are so inclined.