Too much pressure in A/C?

I attempted to charge the air conditioner in my 2001 Mits. Eclipse, I went to Auto Zone, and they suggested I get a can of R134A which included a crappy hose and gauge that now leaks by the way. I located the correct valve and followed the directions in the provided brochure, but the gauge on the can won’t stop reading WELL past 100 on the gauge when it needs to be reading between 45 and 55 according to the brochure. This is with the valve both open and closed. When I detach the can from the car it reads 40-ish PSI.

I also watched a video on this website: http://www…arging.asp which says that if the needle is in the red I need to take it to a repair shop. For what it’s worth, this car has been sitting unused for the last month and a half. Do I really need to take it to a mechanic?

Only if you want the A/C to work.

I can only concour that the readings on the gagues that come with charge cans always read high and I dont know why. Perhaps they are telling you the pressure in the can and not the system pressure? Are you willing to purchase a set of gagues? this will help in your diagnosis.

Was the engine and A/C compressor both running at fast idle while you were attempting to charge the system? Was the compressor clutch engaged and the compressor turning?

Repairing A/C problems is a somewhat iffy proposition for the DIYer and not knowing the high side pressure also presents problems.

However, the first thing you should do is make sure the A/C compressor clutch is actually engaging.
If the clutch is not engaged then then well over 100 PSI you mention on the low side could be the static pressure of the system and this type of pressure would be normal.

With the compressor engaged the low side should drop and the low side reading will vary based on a number of factors; sun shining on the car, ambient temperature outside, humidity, etc.
If the compressor is not engaging then the system will not pull the refrigerant out of the can.

If the compressor is not engaging then this could mean an electrical problem rather than a state of charger problem. Fuse, relay, pressure switch, etc.