Air Condition freon charging question

I have a manifold gauge set for charging R-134 vehicles. I'm trying to find out the proper way to use the gauges myself when charging a system with freon. I know you hook up the low side(blue)hose to the low pressure and the high side(red)hose to the high pressure side on my
vehicle but how do I know what valve side I open such as do I open just he lowside(blue) or do I open just the highside(red) or do I open both sides when charging system with new freon? and also what is a good pressure guage reading on the lowside and the highside of the manifold gauge set?


Before you begin, I would first find check out a few possibilities. Is the air conditioning system completely empty(any pressure)? If so,the leak should be located and repaired first. You will need a refrigerant leak detector to do this job. Is the compressor short cycling(turning on and off)? I would highly recommend getting an a/c beginners manual,pressure/temperature chart,and a infrared thermometer. To put in the refrigerant, just put it through the lowside(car running @ medium idle) in vapor form(read the can as to hold it rightside up,or down). till the low side line is "beer can cold". You can also the can in a bowl of warm water to allow the the vapor pressure to raise slightly. I would suggest that if you partially cover the condenser "in front of the radiator" with a piece of cardboard this would simulate driving on a +90
degree day. As for the gauge pressure, a good rule of thumb would be 35 psi on low and not
over 275 psi on high side would be approx. If you are not comfortable with handling this process, take it to a good auto garage. Pressurized freon can be very dangerous and possibly explode.