R134a PSI Rating?

[Wiz]

My 94 SVX lost its AC pressure and needed a recharge. Probably due to old age. I got a 12Oz can of R134a (no stop leak or oil additives) at Wal-Mart. I got it up to 20-25PSI with the first can and it stopped filling up. I could still feel that there was fluid in the can but figured the pressures must have equalized. To my surprise when I disconnected the first can from the dispenser it dumped out a lot of crap and vented for almost a minute. I guess it was not so empty. Either way I got another just to test the theory and I still can't get it to go past 25PSI even with a new can. The car is running with AC at full blast when I'm doing this. What am I doing wrong or is there something wrong with the car? The second can does not even empty into the car, nothing happens, the pressure stays the same. Its like the car has a valve in it to stop you at a certain PSI rating.

Thanks for any help.
~Ben

PS, if anyone has ideas I still have that second can which is full. And I should have mentioned that its about 53F outside and the AC was running for a few minutes before I tried filling it.

[Tuck]

Was the system completely empty? If any outside air has gotten into the system the water vapor in the air will freeze at the expansion valve keeping any additional Freon from being added. The proper way is to evacuate the system to remove the water vapor before recharging.

Tuck

[Earthworm]

Replacing the dryer would also be recommended if it's still original.