I installed a new well pump and a new breaker... y won't my pressure tank fill up with water

Are we talking about a galvanized pressure tank or a blue bladder tank? If it's a bladder tank they don't fill up with water unless they've gone bad, Back to you. Tom

Stomper, give it a few weeks. The tank, as Speedball pointed out, will never be completely filled with water unless it is bad. But it will progressively accept more and more water as the air under the bladder dissolves into the water. This takes a while.

Yours is a common complaint with new bladder tanks.

We are, of course, assuming you are getting plenty of water in the house at a good pressure. As long as that is the case, then don't worry about the tank.