It is not a situation where the pump is back-flushing the contents of the reservoir up through the filler cap: it is running all the pressurized & cavitated fluid through the high pressure line, through the rack & back through the return line, into the reservoir and
then out through the cap.
One can watch it go down the reservoir as normal liquid ATF and out the low pressure return line as bubbles and pink froth.
The faulty bearing and round seal (if not a leak inside the pump itself, or in the pump housing) is, I presume, where the air is getting into the system.
Just like a leaking O-ring allows air into the pump when the pump is running and ATF leaking out of the pump when not running...
Cheers!