How do i properly discharge a over charged car a/c system at home?

You could just press the schrader valve or loosen the hose fitting on your gauges while attached and open to the system but then that begs the question, how do you know how much to release, especially with R134a.

The best way to handle this is to take it to a certified AC technician with the proper equipment to recover the refrigerent but then the most important part comes next.

You should have a vacuum pulled because you don't know what the previous people did when they overcharged it and you need to make sure the system is clean.

Then the final thing is R134a isn't very forgiving so the refrigerant gets introduced back into the system by proper weight for the system stated by the auto manufacturer.

you can't properly do it at home, unfortunately. are you sure it's overcharged? and if so, by how much?

I know you already know this, but generally, you use ambient temperature and relative humidity to compare with your static pressure to see if its properly charged with the system off. however, even using this rule of thumb won't get you a perfectly charged system.

the machine you need will measure the amount taken out, pull a vacuum to boil any moisture, and recharge to the exact amount needed.

a set of gauges will get you close. and if you're planning on doing this in your driveway or garage, do you really care if you lose .46 lbs into the atmosphere? actually, when you get done charging your system with those tap cans, you have some residual pressure left over that leaks into the air when you disconnect it. about how many people all over the world are using those things? about how much refrigerant are they leaking into the atmosphere in the process.

if it were me, on the side, in my driveway, i'd look around to ensure nobody is watching, and jam a dang screwdriver into the schrader valve. been there and done that! i won't tell, hopefully no one here tells on me. oops.....

I would guess that the little bit of refrigerant you would have to release in order to balance your AC system at the proper pressure is not a significant enough amount to harm the environment.

If you are really concerned about it, then take the car to an auto air conditioning specialist who has the right equipment to capture and recycle the refrigerant without venting it to the atmosphere.

Generally, you don't. The typical shadetree mechanic does not have access to the expensive equipment to do this task. You would have no way to vent the refrigerant into any sort of storage tank. Your only option would be to vent some of the refrigerant into the atmosphere until the correct amount remains. Not really the most environmentally friendly way to do this. At least the modern R134a is more environmentally friendly than the old R12 freon.

No because you have to have the right equipment to do the job properly and AC is done at a specialized shop to have it done right. The fine can be upwrds of 70 thousand dollars so you may want to go down to a shop and have them do it correctly.