Do mechanical fuel pumps get weak?

[72farmer]

I have a 72 that is not getting gas to the carburetor.I took the sending unit out and they are some rust I can see in bottom of tank.I took a old plastic lawnmower tank and plumbed it to the 3/8" steel line going to the fuel pump at the sending unit connections.Of course the lawnmower tank was a lot higher than the pickup on the sending unit.
Turned engine over and got a steady stream of fuel out of the lawn mower tank.So I thought gas tank rust was my problem.Then I hooked a long fuel line to connection on top of tank going to fuel pump.I dropped it down in fuel through the sending unit hole just enough to cover end of the rubber fuel line not getting into rust.
No pumping fuel when I did this.Have you ever seen a mechanical pump get weak?Not lifting fuel out of the tank.