MY OLD R12 SYSTEM IN MY 91 4RUN WAS STILL FUNCTIONING, BUT WAS "DECLINING IN PERFORMANCE"
I WOULD NOTICE A SEVERE DROP IN COOLING, BUT BY CYCLING THE A/C SWITCH OFF AND ON, IT WOULD START WORKING AGAIN (A LITTLE)
I DECIDED TO DO THE R134 RETROFIT.
EVAC'D THE SYSTEM, PUT IN A NEW DRYER, PUT THE RETROFIT FITTINGS ON, BUT WHEN I WENT TO ADD THE FIRST ESTER OIL / FREON CHARGE, THE SYSTEM WOULDNT TAKE IT.
IM ON THE LOW PRESSURE SIDE (SIDE WITH THE RUBBER HOSE), ENGINE RUNNING, A/C ON.
IVE DONE THIS RETROFIT BEFORE ON SOME OF MY OTHER TOYOTAS, AND HAD SUCCESS.
I DO REMEBER DOING WORK ON ONE YEARS AGO (R12 AT WALLY WORLD DAYS), AND I THINK THE SAME THING HAPPENED.
IF I REMEBR RIGHT, IT WAS THE "expansion valve", AND IT WAS A PAIN IN THE A$$ TO REPLACE .

THIS SOUND RIGHT TO YOU A/C GURUS? OR IS THERE SOMETHING ELSE THAT WOULD PREVENT THE SYSTEM FROM ACCEPTING A FREON CHARGE?

 

OOOPS! Sorry, it appears that I am yelling, but I have an injury to my left hand, which makes it hard to use, and type properly, so I just leave caps locks on normally.

Here's a pic of the injury - [Peice of glass broke while I was holding it]

 

Damn! Nice carnage!!!

Sorry I can't help, but my AC is out of R12 and I need it recharged too. I want to stick with r12 though as I've heard the r134a doesn't stay as cold. Why did you opt to make the switch?

 

Greetings,
Nice Pug! Did you try increasing the engine idle like 1500 or so? Try that it usually works. I retrofitted mine awhile ago and had no problems, but mine would not acccept a charge until I had the rpm's up to 1500 or so.
If this doesn't work, your expvalve needs to be rred like you said!
Good luck!