How does a fuel pump work

Used parts guy told me a fuel pump either works at full pressure or doesnt work at all. Is this true? Or can a fuel pump work at low pressure?
My mechanic told me pressure is low at the pump? Can it be a result of bad fuel filter?

The pump's interior is composed of vanes, which are the components which move the fuel. If they are worn, the pump can run at full speed, but if the vanes allow fuel to bypass them, the full capacity of the pump will not be attained. A clogged filter can cause this condition, but the pressure would have to be checked after the filter, an improper procedure... Ray

but the pressure would have to be checked after the filter, an improper procedure... Ray

Are you saying its wrong to check fuel pressure after the filter? If you are you better tell GM their testing procedure is wrong since GM wants fuel pressure checked at the fuel rail. That is why GM installs pressure taps on the rail and not the filter or pump.

What I am saying is that checking the pressure at the rail, after the filter may lead to erroneous readings if the condition of the filter is not considered. Eliminating all of the variables is the only way to find a problem. I read and understand the manuals, they were written by people. Be advised that invariably, these test procedures create results which lead to troubleshooting trees. These trees will gradually eliminate variables, based on the results of the testing recommended. The result is being able to put your finger on the actual problem, with no guess work. No troubleshooting tree is erroneous if it leads to the actual problem. Ray