Some twin systems have solenoid valves well some are a bit more of a eev on suction and discharge if your vacuuming without vehicle running and climate on you will either vac half the system or when you run it and valves open up what ever is in it is now contaminated or worse if your on a modern crysler running 1234yf you pull ur vac you switch on vehicle to warm up switch ac on woops you now have 255 grams of slightly flammable entering your vac pump this is why with the modern equipment on vehicles you have to be carefull you assume ur system empty and it may not be this is where the flammable gas comes from and as far as the pumps go I seen what happens to a class terratrac when some knob vac ed it after assuming it was on 134 when it was on yf the new pump was the least of his issue vac ed it switched on ignition of combine heard the pump change noise then saw the flames come out by the time he got to it there was a lot of fire damage to his equipment and combine