PV not centering-used cpp centering tool

ok, so I bought the

Proportioning Valve Combination Valve Bleeding Tool

from CPP How can I tell if the tip of the tool is actually inside the center of the valve piston and will hold when pumping the brakes? does the bottom flat side of tool have to be flat onto the valve?
When I had the tool on the proportional valve I vacuumed bled the brakes, so fluid was coming out. I closed the rear bleed valves and then stepped on the brakes. the pedal went all the way to the bottom.
So then I decided to gravity bleed the rear brakes but no fluid came out. It looks like the PV tool did not hold the PV centered and the piston moved to the right and stopped the fluid from going to the rear brakes.
Ordered another PV but cannot tell if it is centered. Put centering tool in but cannot tell if it is in all the way to be able to hold during brakes pumping. Can you help? Thanks