The cans of R134a that I use contain stop leak.

 

Alright i'll break it down for you guys, i don't care which one is larger or smaller you can only connect to the low side anyway.
Yes after you remove the a/c 134a and oil from the system you are "supposed" to put the system under a vaccum for 15-30 minutesto boil off any extra refrigerant and get all the excess oil out so it doesn't get caught in the compressor/lines. You more than likely have a leak so re-charging it would be stupid. Using charging cans from autozone or whatever usually contain PAG oil in them so even if you get the right amount of refrigerant in the system without evacuating it correctly you can get too much oil in the compressor and it will cycle on and off preventing the a/c from working correctly. The pressures will look ok on a manifold gauge but you will notice the compressor cycling off and on again because the compressor is saturated with PAG oil. My advice is to take it to a pro with the correct a/c equipment to handle such a job.

 

I used 2 cans of R134a with lubricant and stop leak 2 years ago.
Still working with 30 pounds/ sq. in. pressure on the low side with
A/C running.

 

Well, I put in a large can of the R-134a, but the guage on the line never read any higher than before I started. It continually reads right at the border of the "needs filling" and the "full" shaded areas on the guage, and the pressure never went any higher than 25 or so. But, I did hear a slight hissing from the area around the ports and - not sure if it's a leak or normal behavior with the refrigerant coursing through the ducts....

 

You need to keep the can warm with your hand around it.

 

I did this exact method to recharge the A/C in my 2000 Audi A6 Avant 2.8 Quattro. You just lift up the long plastic cover that is just under the windshield and surrounds the battery (the plastic cover has a mesh like look to it on either side of the battery.) When you lift it up you will see 2 tubes on the passenger side near the battery...the connection that is smaller and closest to the battery is the connection you want to hook up your A/C can to, just like Crazyhawk stated. I hooked up one $5 can of R134a refrigerant that I bought from WalMart, turned the car on and blasted the A/C full blast, then turned the nozzle on the hose hooked up to the can and let the whole can empty out into the system. It worked great and my A/C is now cold (it was coldish before but getting a little warm), but I'm going to get another can from WalMart and put it in because I like my A/C to be ice cold like a commercial freezer during the hot FL summer.