A/C issues on 84 C10 with 5.3 LS

[Dave]

I hate to ask these questions but gotta start somewhere so here it goes

1. How long did you vacuum the system down and what was your reading when you finished vacuuming the system
2. Did you wait till the next day and check readings again, to see if the vacuum held
3. The other thing to consider is the OEM condenser and evaporator were designed to operate for pressures for R12.
134A operates at a different pressure level, meaning the OEM evaporator and condenser will not be as efficient converting 134A back to gas and then to a liquid.
Eliminating the possibilities one at a time is how you'll find the issue.
Lmk what you find