The can will only connect to the right port - the high and low side are of different sizes to prevent people from blowing themselves up.

When the compressor is off, the two sides will eventually equalize in pressure. That number is for all intents and purposes meaningless.

When the system is on, the indicator that it's generally working is a massive difference between the low side and high side pressures.

Unfortunately, modern cars are filled by weight and not by pressure, so those parts store kits really aren't super useful. The pressure guidance they include is based on worst case scenarios; most modern cars will run higher pressure than the kit indicates is safe. Further, AC pressures will be affected by outside temperature, so "system pressure" really isn't a meaningful number. You can use generic R134a charts to get you a sense of where you should be, but they are not conclusive:

Thanks for the info ! Where do iI find that screen exactly?
I guess i will put some in tomorrow and see what happens, a little at a time

24 - 22 CONTROLS ---------------------------------------------------------------------------------------------------------------KJ
A/C HEATER CONTROL (Continued)

A/C HIGH PRESSURE SWITCH DESCRIPTION
The A/C high pressure switch controls both A/C compressor clutch engagement/disengagement, and electric cooling fan operation when equipped with the 2.8L diesel engine. The A/C high pressure switch is mounted on a fitting located on the A/C discharge line near the A/C compressor. The A/C high pressure switch turns off the A/C compressor if the refrigerant system pressure exceeds 3100 - 3375 kPa (450 - 490 psi). The fitting for the A/C high pressure switch on the A/C discharge line is equipped with an O-ring seal and contains a Schrader-type valve, which allows the switch to be serviced without discharging the refrigerant system.

OPERATION
The A/C high pressure switch is connected in series electrically with the A/C low pressure switch between ground and the engine control module (ECM). The contacts in the A/C high pressure switch open and close causing the ECM to turn the A/C compressor clutch on and off. This prevents A/C compressor operation when the discharge line pressure approaches high levels, and also reduces electrical surging from A/C clutch engagement.
The A/C high pressure switch controls the electric cooling fan operation by monitoring refrigerant line pressures. When the A/C discharge line pressure rises above 1900 to 2200 kPa (280 to 320 psi) the fan will turn on. The cooling fan will turn off when the A/C discharge line pressure drops to 1600 kPa (235 psi).
The A/C high pressure switch contacts open when the A/C discharge line pressure rises above 3100 to 3375 kPa (450 to 490 psi). The A/C high pressure switch contacts close when the A/C discharge line pressure drops to 1860 to 2275 kPa (270 to 330 psi). The A/C high pressure switch is factory-calibrated and cannot be adjusted or repaired. If the A/C high pressure switch is faulty or damaged, it must be replaced.

DIAGNOSIS AND TESTING A/C HIGH PRESSURE SWITCH
Before performing diagnosis of the A/C high pressure switch, verify that the refrigerant system has the correct refrigerant charge (Refer to 24 - HEATING & AIR CONDITIONING/PLUMBING - STANDARD PROCEDURE - REFRIGERANT SYSTEM CHARGE) For circuit descriptions and diagrams, refer to the appropriate wiring information. The wiring information includes wiring diagrams, proper wire and connector repair procedures, further details on wire harness routing and retention, as well as pin-out and location views for the various wire harness connectors, splices and grounds.
(1) Disconnect and isolate the negative battery cable.
(2) Disconnect the wire harness connector from the A/C high pressure switch.
(3) Check for continuity between both terminals of the A/C high pressure switch. There should be continuity. If there is not continuity, replace the faulty A/C high pressure switch. If there is continuity, test the A/C high pressure switch sense circuit (Refer to 8 - ELECTRICAL/WIRING DIAGRAM INFORMATION - DIAGNOSIS AND TESTING). Repair as required.

REMOVAL
NOTE: It is not necessary to discharge the refrigerant system to replace the A/C high pressure switch.
(1) Disconnect and isolate the negative battery cable.
(2) Disconnect the wire harness connector from the A/C high pressure switch (Fig. 20).
(3) Remove the A/C high pressure switch from the A/C discharge line and remove and discard the O-ring seal.

INSTALLATION
NOTE: Use only the specified O-ring as it is made of special material for R-134a. Use only refrigerant oil of the type required for the A/C compressor.
(1) Lubricate a new rubber O-ring seal with clean refrigerant oil and install it on the A/C high pressure switch fitting. (2) Install the A/C high pressure switch onto the A/C discharge line. Hand-tightened the switch securely. (3) Connect the wire harness connector to the A/C high pressure switch. (4) Reconnect the negative battery cable.
(2) Install the A/C high pressure switch onto the A/C discharge line. Hand-tightened the switch securely.
(3) Connect the wire harness connector to the A/C high pressure switch.
(4) Reconnect the negative battery cable.