Why did the needle go back to 33 psi after two 12 oz. cans of freon?

Answer 1

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The vapor pressure over a liquid is the same no matter how much liquid you have.    Freon in one section is liquid and if you measure the pressure there it will always be the same.    Measure the pressure where the freon is gas.  

Answer 2

When the AC compressor cycles off the pressure on the low side drops. If the AC constantly cycles on and off the system is still in need of charging. If the temperature of the air coming out of the center vents is 42 degrees the charge is good enough.