I have never heard of pulling a vac on the cooling system.
since it is meant to be under 12 to 16 pounds of pressure, I have to ask what was the point?
Some items may not hold pressure /vacuum if they were designed for the opposite.
if you put it under 10 pounds of pressure, does it hold?
Very common to pull a vacuum on a cooling system before filling. Especially on the newer stuff, in some cases the cooling/heating tubes are so small in the heater core for insistence it is impossible to purge all the air out of the heater core without a vacuum.
I would try to pump it up and see if you can see/hear where it is leaking next.
Good luck.
