New fuel pump, the car's performance suddenly improves when the tank drops to 1/4 full. I'm yet to order a fitting for a pressure test

 

I don't know, but don't let it get below 1/4 tank.

 

Why is this?

 

Long term, pumps are cooled by fuel, habitually letting the level get low will cause the pumps to get hot and fail prematurely.
More damaging even is the possibility of fuel starvation during lateral turns that can cause detonation and can actually damage your engine.

 

 

Yes I'm very aware of that, and it is why I'm confused with my situation. Btw, can a clogged fuel vapor filter cause performance issues?