Years ago I worked in a shop that had a vacuum varnishing tank and I remember enjoying its use, namely because of all the valve turning, rattle guns and pneumatics involved.
There were two tanks, a square one on the shop floor with two hinged lids on top and a cylindrical one with sphere ends with the top just sitting above floor level. The latter was the vacuum tank and presumably cylindrical/spherical because of the superior strength under pressure/vacuum.
These were very large tanks, able to hold very large stators, armatures, wound rotors and transformers.
The varnish was kept in the square above ground tank which was often used to dunk stuff into prior to being baked in a large industrial oven, and had a valved pipe going to the bottom of the vacuum tank.
You would undo do at least 2 dozen 24mm bolts that held down the spherical lid. It was attached to a swinging boom with a pneumatic ram to lift it.
Once the item was inside the vacuum tank, you would measure how high you wanted the varnish to come up so you knew how much to suck out of the square tank. You would then swing the lid into place, line up the hold down lugs, lower the lid and then go around with the rattle gun tightening the bolts.
You would then close the air inlet valve to the tank, open up the suction line and turn the pump on.
Once the gauge read the right amount of suction you would open the valve on the pipe going from the square tank and simply wait until the varnish level dropped the pre-measured amount. At this stage you would shut off the varnish line.
The air inlet line to the vacuum tank would be opened, equalizing pressure and then feed filtered compressed air into the tank. You would leave it pressurised for an amount of time determined by the type of winding. Formed coils (taped copper section) would get a longer stint than normal wire wound coils.
When the time was up you would re-open the varnish line and the positive pressure would push the varnish back up into the square tank. When the varnish approached the original level in the square tank, you would close the lids and get ready to close the valve as soon as you heard it bubbling. Needless to say occasionally someone would walk away and if the tank was pretty full inevitably you would end up with varnish bubbling out of the tank.
Personally I had never witnessed this but evidence of it was ample by the amount of dried varnish on the surrounding walls