Ah, c'mon Dyoel182, clue in, it's suction PRESSURE. The only vacuum you will ever see in a normally working system is when you are vacuuming the system down before you recharge. It can still be suction because the low side is much lower than high side. Can of freon has about 70-80# in it, that's what allows the gas to flow into suction which runs from 20-25# low to 50-55# + at the suction high. That is PRESSURE, NOT VACUUM. That's why you don't necessarily have to vacuum down system if you leak some refrigerant out. Air at normal ambient pressure will not leak into a system that has higher pressure than that, talk to the physics teacher. If you are close to zero in system it needs vacuum, but much higher than that, no. The residual pressure will keep AIR OUT. A good indicator of whether you need vacuum or not is the a/c clutch. If you have leaked enough so that clutch will not come on any more, you need to vacuum system down. Clutch will shut down around 20-25#. The guy above mentioned 45# suction high, that's exactly what I was talking about in saying that the gauge on can USELESS. He could be running 130-140 ish on high side, and by adding coolant go all the way up to 250-275 on high side while only adding maybe 10 more pounds (to 55) on the low side. There's a stark heavily tilted ratio there, charging to low side numbers is worthless. The high side is how you know where you're at. Also, everybody talks in entire cans as in "can I put another can in ?". You don't go by entire cans, someday that urge to squeeze that last half can in is going to cost you big when you rupture a line or condenser core. You use the high side gauge, hit the proper number for temperature/pressure, verify proper cooling and prepare to lose that last 1/2-3/4 can of refrigerant. That's just the way it works out. There's a proper amount to put in system, its' performance decays with either too much or too little. To help prevent loss of freon like that, I will often go on a 'charging party" where I charge up all 3 of our running cars at one time to maybe save part of a can wasted.