Testing.
More Testing.
Even More testing.
Running the pump for longer with only the pump and PSU on, no other components, until I'm satisfied that nothing leaks.
Verify and Validate every change this way. Even small ones.
"There are no small fixes."
Bring more disciplined about never ever reusing any fluid that comes out of my case. If that means keeping 5 bottles on deck just in case, that's fine.