Ahhhhhhhhh.
Start from the beginning you have a r12 system.
You can not add r134 refrigerant period.
Now if you want to convert to r134.
You must replace the accumulator, drain most of the oil and measure it, replace it.
It will not mix with the r134a.
Replace may be the exp. valve.
And may be hoses, compressor etc. If not working.
Vacuum the system down ck for leaks and fill.
This is the minimum.
If you don't have the knowledge, and have the necessary tools stay away.
One more thing it is not legal to vent the a/c system.
Regards.
aam.