if somebody pumps too much, in order to prevent rubber piston from damage, at certain vacuum pressure (for example 20 kpa), it will automatically release the pressure. I have never used it in past 4 years.
using it is very safe, there will be no spill.
there is a tip - after you finish the job, the tube (color is changed from clear to black) will be oily. the oil will take time to creep out. that is where contamination takes place. i made a tool myself - use a piece of 1/2" PVC pipe about 5 feet, cap one end and let open end up, put the oily tube inside, lay at side you are good.
