Lift pump pressure

[LKothe]

I've got a 98 1/2 slush bucket 24v.
Used to be that when I would start it, the lift pump put up about 10# for 3-4 seconds
and then drop back to zero. Pre prime I'm guessing. Then when the truck started, the
pressure would go up to a steady 18# and stay there no matter how hard I pushed it.

Now it doesn't go up to 10# when the key is turned on but Will go to 18# when the
truck starts. I'm kinda sure it is a sensor but which one. Thanks...