Help Confirm my A/C system readings

Good morning everyone,

I have a 2003 cobra svt that has been having A/C issues. I think I know the problem but I would appreciate some confirmation before I blow it up.

Engine running, A/C on max and fan on high.

Using the freon recharge kit gauge, low side pressure indicates ~50 psi before the compressor engages. When the compressor engages low side pressure drops to ~0 psi, the compressor disengages and pressure rises back up to ~50 psi.

I added R134 but did not see any noticable change in the pressure with the compressor running but with it off the pressure rose to ~55 psi.

I believe the system is working properly based on that and is just extremely low on freon. The pressure rises back up due to high side pressure bleeding over to the low side. I have read that low side pressure should be about 35 psi and never more than 50 psi, but I can't find anything that says if that is with the compressor off or on.

I intend to get a high and low side pressure gauge set and add more freon but want to make sure I'm on the right track before dropping more money on this one.

Thank you,
Ken


(I do not know much of the history of this car. It was given to me by an old friend, it's in decent shape but it feels rude to ask him it's maintenance history.)