You might find this interesting: After flushing the fluid on my 1997 F150, I wanted to be sure all the air was completely removed so I pulled a vacuum on the reservoir. I used an orange transparent pill bottle to cap the top of the reservoir. The round opening of the reservoir and the pill bottle matched up for an air tight seal. I drilled a ¼” hole in the bottom of the pill bottle, inserted ¼” vinyl tubing and connected the other end to an engine vacuum line pulling 18 inHg.
When I placed the pill bottle on the reservoir there was an instantaneous fog in side, but no bubbles coming up. It looked like vapors, but it also could have been condensation on the inside or both. It remained dense for about twenty seconds then began to dissipate until it was gone. I think it was moisture vapors out gassing from the brake fluid, same as pulling a vacuum on an AC system to remove moisture. Whatever it was, I don’t think it should have been there and it makes me wonder just how dry was the new brake fluid. This proves nothing except that I saw vapors that I could not identify, but it was certainly some type of out gassing.
From now on when I service my hydraulic systems, I am going to flush the lines dry and then pull a vacuum on the empty system to remove moisture. If it is done on the assembly line, it is probably a good idea. After filling it with new fluid, I’ll pull a second vacuum to boil off any moisture that may be in the new brake fluid. New fluid may not be as dry as it should. It all depends on the quality control of its manufacture and we all know how things slip past.
To pull a vacuum I tap into my engine's manifold vaccum. I have use simple plastic food container lids with a ¼” hole drilled and a ¼” vinyl line inserted. The vinyl line fits snugly in the ¼” hole and does not need anything to seal it. Sometimes the hose does creep inward when the vacuum reaches maximum. These simple plastic lids work pretty good sealing off the smaller openings up to about 1 ½ inches in diameter. This is a very simple and effective method for pulling a vacuum. I used the pill bottle hoping to see bubbles and was lucky it matched up and sealed. Just for fun, I pulled a vacuum on a plastic 64 oz Hellmann's Mayonnaise jar. It took about 10-seconds to flatten it.