I am an automotive AC technician on the side
Did you pull a vaccum on the system for 45minutes to an hour before trying to charge the system? ANY atmosphere in system is explosive when pressured with R134-A. At the very least you will get very high readings. I made a vaccum pump out of a refrigerator compressor which does the trick, but a shop can do it for you.
Putting a vaccum on the system will cause any atmosphere (water, humidity) to "pop" and vaporize and be expelled from the system as a gas. Then the first can of freon is just "sucked" into system allowing for the pressure switch to initialize the compressor clutch.