May I take a moment to express my admiration to auto makers for not putting sight glasses or high side service ports on their air conditioning systems??
Sure, while having a gauge on the high side while charging and testing the system isn’t an absolute necessity, it helps a lot!! It can warn you if the system is overcharged, the orfice tube or thermal expansion valve is obstructed, or of bad cylinders in the compressor (wild gauge vibrations).
Excessive high side pressure can pop the compressor or even cause an explosion.
With only a low side port, you might as well just be using one of these stupid “pound my system in the ass” cans.
Believe it or not, I’d actually say the TOP of that can is kinda alright to use, if you have a one port system. Guilty parties I’ve seen this from to date: Mazda, Ford, and Volvo.
The problem is the can. These kits come with a can of pure death: the can injects additional compressor oil into the system as well as a “stop leak” compound.
The additional oil can really screw things up. It’s not compressible. There’s a sump in the bottom of the compressor that collects it and splashes it on the swash plate drive and backs of the piston rings. Overfill and it will enter the cylinders and cause liquid slugging which will bend and break parts. BANG!!!
Stop leak is just vile. The most common form is a substance that soaks into, swells up, and structurally weakens elastomer O-ring seals. There just aren’t that many in a car a/c system and they are easy to replace. The proper procedure is, if you have a leak, have the system recharged and a UV dye injected. This will make the leak glow and it can be detected with a blacklight. The o-ring can be changed after recovering the refrigerant. My own experience has been that a compressor seal failure tends to follow the use of one of these, leading to a slow expensive leak.
Okay, so if you still really want to use one of these goofy can top kits….
Do not use the can with stop leak and oil. Get a can of straight up R-134a ONLY.
Put a digital thermometer in a dash vent and place it where you can watch it while charging. If you observe a rise in temperature, STOP, turn off the ac system immediately, and go confess to your local actual professional what you’ve done.
NEVER TURN THE CAN UPSIDE DOWN. You’ll slug the compressor.