I just did a Search on the token "fresh water tank filling" and got 16 hits. Other similar search token would produce many more since this issue is so often reported.
In one thread, I found where our friend Flybob wrote this:
Originally Posted by Flybob
Little more detail:
Disconnect city water.
open a faucet to release pressure.
turn on water pump and allow water to run for 10 secs.
turn off water pump.
reconnect city water.
You should be OK. Do not use water pump with city water connected. It can cause the check valve in the pump to seat improperly.
Also, you stated "my low point drain is running water like crazy"... as if you have a single low point drain. Most units have two low point drains - one for cold water pipes and one for hot water pipes... plus units generally have a fresh water tank drain. Which of these three possible drains is running water like crazy? Maybe the cap is not screwed on tightly.