[Trans-Am Jim]

The puke tank should not be pressurized, and have a vent at the highest point.

Pressure is regulated at the expansion tank with cap. Cap has two-stage valving.

It works thusly:
when pressure in the expansion tank (in my case, could be a radiator) exceeds 18 lbs. (that's my cap's setting; others may differ) the cap opens and vents excess fluid into the puke tank. I have my puke tank overflow set up to dump on the headers, evaporating any fluid venting from the puke tank.

When the car starts cooling down after shut off, a vacuum is created, opening the secondary valve on my cap, allowing any fluid in the puke tank to be sucked back into the system. The puke tank allows this, because it is vented, not sealed.